Best places to stay in Europe to avoid the summer heat?

If it's hot in Scandinavia, then western and central Europe are likely to be a blast
furnace. So the odds are more in your favor in any of the cities you note, but not
complete certainty of moderate temps.

It's not clear how much you want to sightsee and how much you just want to relax in
a temperate climate. The answer to that question might help decide how big a city
you need to go for.

Stockholm is surrounded by water, for instance, and there is lots to see. Tallinn, though,
is less Western and more unique, but smaller. And it's a lot cheaper.

In the mountains, it can still be hot (80's) during the day, but it cools off much more
dramatically in the evenings.

Is language an issue?

Of all the Scandinavian capitals, Copenhagen would be my choice for a month in July or August. Lots to see and do in the Danish capital. Also, Denmark
is a great place to visit with a baby. You might even split your time between Copenhagen and cities such as Odense, Aalborg or Aarhus.

The Scandinavian countries are the most likely choice for cooler conditions in summer but these days with global warming, who knows if that's still true. They tend to be somewhat more expensive to travel in than other areas of Europe. As you're travelling in July or August, it will likely be very busy and crowded everywhere in Europe, so booking accommodations or whatever well in advance would be prudent.

Which destination to choose will depend to some extent on your interests and what you want to see or do while in Europe. Are you interested in museums or other city sights, or do you prefer walks in the country? The Rick Steves guidebooks for each area will provide lots of ideas for things to see and do. If the latter, you could consider the Trentino-Alto Adige/Südtirol area of northern Italy as there's some incredibly beautiful areas to hike. Temperature charts online show temperatures of 22C high and 9C low in July and August. The Berner Oberland area of Switzerland is also an area to consider, as the mountain areas especially seem to be a cooler than the more southerly parts of Europe. Temperature charts for that area show 17C high and 5C low.

Good luck with your planning!

I will throw a curve ball and suggest that you visit Australia. It is winter there in July and August, but gardens think it is spring in August in Sydney. If you are looking for mild winter temperatures, try Queensland. Most tourists get a kick out of Cairns.

Lavandula

How important is being in a "city", and how big city do you need? Because there's a number of places where you can go without worrying about the temperature if you'd be satisfied with someplace less accessible. Big cities are heat traps, this is common knowledge. Look at places where you have water to moderate the heat, and preferably some wind. This is why ocean front property is so desired.

So, consider Ireland, Scotland, and Jersey, all of which are English speaking locations (more or less). Iceland is good also in summer.

But I'd head to the Frisian Islands off the coast of the Netherlands and Germany. Look at Borkum, Juist, or Norderney on the German side, or Schiermonnikoog or Hollum in Holland. These are places where the primary mode of transit is the bicycle and the pace of life is slow. They are also extremely popular at the time of year you're planning to go so you'd need to be making reservations ASAP.

A cabin on the beach, no cars, good food, easy walking, and good weather makes for a really nice break. At least, for me it does.

My optimal choice climate-wise would be Iceland - but you just need a car there.
Generally speaking, mountains are suffering from being less hospitable, so you will have trouble finding a city large enough at an elevation high enough.
Now when I want to flee from the summer heat in her, I tend to look north. The Baltics and Scandinavia have worked for me, but it can definitely go to 85 or so occasionally (though only for the day).
Norway (Bergen or Oslo) is likely the safest bet - but Norway is also the most expensive.
Tallinn or Riga were fine last August, but I would guess more around 80ish. Possibly even safer might be Dublin, Edinburgh or Aberdeen in that regard - and also maybe easier to reach. At least the latter also has a very convenient city beachfront.

Seconding a bit the ideas pondered by KCG before, going some place along the Wadden Sea shore (Denmark, Germany, Netherlands) might be a reasonable idea as well: At the coast itself, it will be rarely higher than 75-80 due to constant wind etc., and your kid can play in the sand all day. And if it is cooler, the hinterland is easily reachable by public transport - it is much more densely populated than Scandinavia or the Baltics, and thus the transport is likely better, with denser schedules etc. Plus trains are much easier with a stroller than busses (arguably the most convenient, even more than a car). My sister lives in Hamburg, and she travels to towns at the coast whenever it feels too hot in the city. On the German part of the coast, I would assume places like Emden, Cuxhaven or Husum to work well - I am sure there are nice places in the Netherlands or Denmark as well, but I am less familiar with these. AC seems to be common enough there - just checking at a website showed well over 100 in Cuxhaven.

the average temperature would be around 75°F / 23°C

Unprecisely request. Do you mean day average temperatures?

Are you aware that Europe widely has in summer more different day and night temperatures in summer? Therefore the month average temperature says not much. Example: Berlin has 23° C average in July and August but daily peaks can get up to 35 or 36° C at hot days - plus sun.

If you look at newer climate tables of Copenhagen (1991-2020) for example you will see also 22° C average in July and August. Closer to the sea they have less extreme peaks because the sea cools down if right wind. Copenhagen is a good choice but be aware about the peaks.

Info: you will find week periods also close to polar circle (Northern Norway, over 1,000 km North of Copenhagen) with day peak temperatures wide over 30° C (read this thread).

btw: strictly defined (geological) Copenhagen and Denmark are not part of Scandinavia but for sure one of the Nordic countries. The name-giving Scandinavian Mountains are the longest mountain range in Europe and located only in Sweden and Norway - the few hills in Northern Finland are not considered.

Greetings from this night's -8° C - around 250 km South of Copenhagen.

It's not possible to predict the weather accurately far in advance. To some extent you will be "playing the odds."

Last July, we were in Shetland (out in the North Sea, off the north coast of Scotland). The weather was lovely mostly, with most days in the 70s and 80s with clear (intense) sunshine. Locals were giddy and gobsmacked by the sunshine. I got a moderate sunburn (I burn easily - like the people there - some thought I was a local). Many of the the locals appeared to be badly sunburned to me. One old man (clearly a local) told me he had never worn short pants outside before - he was smiling, but red as a cooked lobster. After Shetland, we flew to Bergen, Norway. It was HOT there, too (the immigration officer who stamped our passport was grumpy and complaining about the heat, in the 80s). Our hotel room in Bergen was hot, and had no A/C, just a fan. It wasn't awful (not like Sicily...) but local people seemed to have had more than enough of warm sunshine.

So no matter where you go, you take your chances. If you MUST have cool weather, you may have to choose a non-urban location. Anywhere with a seacoast, if you go far enough north, will be a good bet. We loved the Faroe Islands. Western Ireland, the north of Scotland...plenty of lovely places are good bets.

my initial response is Shetland or Orkney in far northern Scotland, or Iceland, or Nuuk in Greenland.

I spent much of the last two summers in Scotland. On some days, I needed a light jacket, It's rare to go above 75 degrees F. When it does, my local friends complain about the heat,

The nice thing about Scotland is that there are numerous easy day trips that can be made by either train or bus,

If you need/want a/c, then look for apartments that have a/c.

I am from South Georgia where is gets hot in the Summer. I have been to places like Spain, Italy and Greece in the Summer, no problem. However, I know others aren't happy with high temps.

I lived in Augsburg, Germany for four years and it was rare for temps in the Summer to go over 90 degrees. The locals would freak out then, because most of the time it wasn't hot.

If you want to for sure go to a non-hot place, I suggest Scandinavia, the UK (especially Scotland), Ireland, Iceland, Switzerland and Australia, Chile or Argentina.
Helsinki, Oslo, Stockholm, and Tallinn should be fine in the Summer.

However, I’ve heard that many apartments don’t have AC, and without AC it can still feel quite warm and uncomfortable.

One major difference to be aware of is this: In the Southern US it is common for it to be both hot and humid. In Europe most of the time when it is hot it is dry. That makes for a vastly different feeling, and is also the reason why AC is rarer. The benefit of AC is that it lowers humidity. I am quite happy that many jurisdictions now do not permit AC to be set to low. When I first went to the Middle East I got a cold because of the omnipresent air conditioning...

Another important factor: Houses are more often of stone in in Europe. So in most of Europe, during the few hot days we have we just keep indoors cool by keeping the windows closed during day time, and opening up at night. Especially in the mountains you can count on it to cool down substantially at night, so the occasional heatwave is perfectly survivable without AC.

Considering cities as you request, the Nordic suggestions such as Copenhagen are good, but here is another one: Manchester, UK. Average high in July is 69 F. You can fly there directly from the US (3d busiest airport in the UK after London Heathrow and Gatwick). It's a big and busy city with good transit and interesting attractions, walkable, very cosmopolitan but much less touristed and more budget-friendly than London. It is within easy daytrip or weekend trip range of beautiful nature (Peak district, Yorkshire dales, Snowdonia in north Wales), other attractions (seaside and castles of north Wales), and other very interesting cities (Chester, Liverpool, York). Northern English people are quite welcoming.

Living in southern Sweden during the summer I never felt like I needed air conditioning like I needed it back home in Spain.

The UK (even here in the south) and Ireland really don't get very hot in the summer. We might get the odd week or so where it's in the high 20s C, maybe even 30C one day, but it doesn't last. We don't have AC and have never wanted/needed it. Summer really is the best time to be here- and London is extremely walkable and nature is an easy train ride away (as well as all the lovely parks etc within and on the edge of London).

Yes, Cat puts it well. A month is a long time to spend in one place and London has a lot to keep one entertained.

It generally pretty dry all through the summer in London. After about 20 years of living here it still surprises me sometimes how long we can go without rain in the summer months.

Scotland was my first thought. It's generally a few degrees cooler than London and the south east in summer. The chance of rain, even in the middle of summer, is much higher, especially in the west of Scotland. There can still be chilly days well below 15c (59F) in the middle of summer in Scotland, which are less likely in London.

"Attractions" is a very vague word.
My suggestion is to think hard about what you want your days to look like. Are you the type of people who need to be going and doing all the time or are you looking to relax and slow down? Do you intend to cook most meals or eat out or a mix? Is your toddler going to take a 3-hour in the middle of the day? If so, what will you and/or the other adult(s) do during that time? Can/will the child nap quietly in the stroller while you're reading all the placards in a museum? What time will the child go to bed at night and will they be able to sleep if it's not dark out (it won't be if you're in Ireland, UK or a Nordic country)? Someone else asked about the language. While most people in the Nordics do speak English, you'll still be dealing with a language barrier in lots of situations. If you're looking to "settle in" and live in a neighborhood, you might prefer the UK or Ireland, where they speak a form of English that is more or less understandable. What if you need to see a pediatrician or (God forbid) visit the emergency room? Is that going to break you financially?
All that said, Oslo has a good variety of very accessible natural areas, coastal and more hilly/mountainous. I don't know whether you would feel there are enough attractions there, of course. And it can get hot, and they're not prepared for it. As others have mentioned, AC isn't common in Northern Europe because the need for AC isn't common. And you won't know whether you need it until you need it. One way to mitigate that, regardless of what city you choose, is to look for accommodation that is outside the city center, preferably in an area with lots of trees or near the shore. Then you need the public transport to get to the attractions. Or find a place with AC.

If you want to see something different, I would suggest also heading towards NW Germany towards the North Sea, seeing the historical Norderney in particular , and Cuxhaven

A decade ago or so, I did an afternoon trip from Hamburg to Cuxhaven....interesting to be sure.

As suggested , Husem in Schleswig-Holstein would also be worth your time, as well as the entire region.

Another non-European option is Vancouver BC. Direct flights take about 4.5 hours.

David and Nigel both mentioned Shetland. I was in Shetland about the same time as David last summer and was shocked at the warmth. I’d been in August 2023 and was quite chilly. In fact, last summer the 2 weeks before I was on Shetland, I was on Orkney and was wearing my puffy vest under my rain jacket. I’m pretty cold tolerant but the wind on those islands adds some cooling.

Still, if I were looking to escape the heat Orkney/Shetland is where I’d head.

Plus…Puffins!