Europe in summer - options

How about Denmark and / or South Sweden?

If you do a car round trip from / to Hamburg you can include the paradise Miniatur-Wunderland for boys of all ages.
Ferry rides, huge bridges, long sand beaches, old towns, WWII submarines, Viking villages and boats, Danish hot dogs with red sausages, Danish soft ice, original Legoland, nice old towns, castles and palaces.
Some other tips available.

This is a great suggestion!

Wi nøt trei a høliday in Sweden this yër ?

See the løveli lakes
The wøndërful telephøne system
And mäni interesting furry animals
Including the majestik møøse

Kidding aside, Hamburg is fun, Haithabu and up into Denmark there are Viking sites, Billund has an appeal for Lego enthusiasts and small town lovers, and of course there are tons of coast and castle opportunities along the way. Then crossing into Sweden you can do more nature oriented stuff as well.

Consider Latvia.

Montenegro.

I feel like maybe we are hearing hoofbeats and yelling out zebras, alpacas, and robotic cows?

Central western Europe is a massively prominent area for tourism. And deservedly so.

The Netherlands is a wonderful place to visit, and phenomenally fun for children. Roll Efteling or Dunedin (both remarkably great themed amusement parks) into visiting Amsterdam and some small cities. Ride horses in the country. Ride bicycles through UNESCO windmill areas. Visit Belgium as well.

Or northern France. They have this town called Paris, have heard it's worth a look ;) And then the rest of France has the most geographic diversity of any country in Europe. You can go literally any direction from Paris to a fantastic vacation of many different types.

Germany, Switzerland and Austria could occupy your next 10 trips no problem.

Austria is a wonderful family summer trip. One of the world's greatest, most livable big cities Vienna, drop a glass dome over it and call it a museum Salzburg, gorgeous warm mountain foothill lake resorts. You can spend half your time soaking in culture, and the other half in flip flops staring at beautiful mountains at lakeside water playgrounds. And Austria caters to children big time - there's never not specific stuff for kids to do. Go through Linz and take the cog train up to mine tunnels decked out as a glowing magical dwarf kingdom.

Go to the Swiss Berner Oberland. It's family vacation heaven.

Go to Berlin. Stay in Potsdam and enjoy a posh little family town with every good life amenity you could possibly want, massive imperial gardens to stroll and rent bikes, surrounded by a network of lakes and rivers (stay on a houseboat if you please) for swimming and water recreation. And then take the train 20 minutes into another one of the world's greatest cities. Then zip down to Bavaria for beer and castles and baroque. Or up to the north sea coasts. Or over to Prague.

Endless options!

The reason why I did not recommend Berlin area is the UEFA soccer championship which makes German cities and neighbor areas very crowded by tourists and - hopefully - a little bit positively weired.

HowlinMad got my idea: Laboe with submarine and beach, Haithabu, Rømø beaches and and Ribe old town before Billund. After that a short stop at Aarhus (old town museum, Ebeltoft with large ship, - then Roskilde viking ships, then Sweden (maybe by HH ferry from Helsingborg - super castle). Back via Oresund bridge ferry to Puttgarden, last stop in Lübeck. Alternatively to Aarhus and turbo ferry Odense and Storebelt bridge.

Love those ideas.

Hank - Austria in mind

My sister in law who has 12 children and lived just outside London for many years says her children's favorite place on the continent is Mont St. Michele. We've been twice with our youngest when he was 7 and again when he was 13. He loved it both times. His next favorite spots would be Rothenburg and Salzburg.

Agree with Howlin & Hank, go north or go up into the mountains, summer will be quite warm & unfortunately more crowded. Personally, the Danish / Swedish route appealed the most because it will be scenic, off the beaten path, lots to do for kids, midnight or near to it sunshine, beaches, small villages. Maybe have a look at the cost of accommodations on Booking.com, not to book there but check prices. I'm betting Sweden won't be any more expensive than Austria, if not cheaper during the summer. (Even if not on a budget, checking hotel prices will give you an idea of demand, ie crowds.) Good luck!!

Montenegro. You can spend time on the coast and enjoy a lot of water activities like prrasailing, jet skis and Deep sea fishing, or just hang out on beautiful beaches and then go 2 hours away to the mountains zip lining over river canyons and river rafting, hiking. All for about 60% of the cost of the same in most of the west. Culture ranges from AustoHungarian in Perast to Venecian in Kotor to Ottoman in Ulcinj.

Herceg-Novi https://www.youtube.com/watch?v=dOqLXoeE_o4
Cetinje https://www.youtube.com/watch?v=r6RwWDPe9M8
Perast https://www.youtube.com/watch?v=fd6ErEHPHmg
Budva
https://youtu.be/gR0I_ka53Tc?si=2IWJiVqJ3zozvXsB
Uncinj https://www.youtube.com/watch?v=LaPcThRQdi4
Fly Fishing https://www.youtube.com/watch?v=MpnDIoqvfbo
Tara River Canyon
https://youtu.be/9aGMZB8hB5Y?si=K1XOiR1knGNY71sJ
Rafting
https://youtu.be/LI16mZETyi0?si=T8wtcBpBGMHTSJD2

I'd do Austria--flying into Vienna. Taking a train to Salzburg and a train down to Innsbruck. You could rent a car as you leave Salzburg and see some of the most magnificent mountains on earth--turning it in at Innsbruck.

Then you could take a train 2 hrs. or so up to Munich and fly home.

Postlingenberg and grotenbahn (spellings likely butchered sorry) in Linz. Fantastic little excursion with kids.

https://tripbytrip.org/2022/10/10/postlingberg-grottenbahn-in-linz/

Your lakeside fun time might be spent in the Salzkammergut lake district area right behind Salzburg. The area was an imperial retreat for centuries. Many many (many) great spot to rent a flat and then explore. Consider St. Gilgen. It's a lightly overlooked lakeside town with tons of charms and just the right amount of amenities. The last time I was there it was being used as a movie set for a period piece. Mini golf, municipal lakeside swim play park, child level always clipped in (aka via ferrata) climbing routes, ferry terminal for exploring, summer mountain coaster nearby, brand new cable car from town whisks you up into the alpine. Or Strobl at the other end of the lake, just a bit less atmospheric, but also a bit deeper into the region, quite practical and user friendly.

https://www.theworldinmypocket.co.uk/charming-things-to-do-in-st-gilgen-austrias-prettiest-mountain-town/

Vienna of course. Sorry for burying the lead. Vienna can rightly be defended as the easiest big culture city to visit in Western(ish) Europe. Seat of a centuries long massive empire with all included spoils, it hangs with Paris, Venice, Rome (if maybe a hair more junior). But it's also quite homey - regularly awarded most livable large city in Europe, and displays much folk culture as well. It's both a mind blowing cultural experience and relaxing. You can tour some of the world's great museums in the morning, and then swim on the Danube island in the afternoon. Catacomb tour of a massive cathedral and then walk to a big historic amusement park. Vienna has the family
friendliness characteristic of Austria, kids are a feature not an after though.

A last one w/ family focus, from Salzburg you are close to the Porsche toy museum just across the border in Germany. Yes, that Porsche, and yes one of the family assembled the largest collection of german toys in the world. There's a ride-on mini train for kids, and one if the largest and most complex miniature railways in Europe. It's a fun kid thing that isn't pure sugar but has some mental nutritional value as well.

https://youtu.be/h-xMpSkkni4?si=qbC0vkFFYgY0_m2M

Much more great stuff for families in Austria - so many good choices :)

Have fun!

Luxembourg and vicinity

Wi nøt trei a høliday in Sweden this yër ?

See the løveli lakes The wøndërful telephøne system And mäni
interesting furry animals Including the majestik møøse

A moose once bit my sister…

Denmark/southern Sweden/northern Germany is not a bad idea. Plenty of things to see and for an early July trip the temperatures will probably be more reasonable than southern France.

This hasn't yet been suggested, but I'd consider Poland, is really undiscovered for American tourists. The country has undergone a transformation in the last few years using its strong economy for upgrading a lot of infrastructure. There are many places to see in the country from the wild Baltic coast in the north to the rugged Tatra mountains in the south. Plus there are loads of charming medieval towns in villages in between all the major cities, many of them without any major tourism. I'd say Poland is on par with Germany or Austria, in terms of things to see and do and depth of culture and history, and for probably half the price.

@Badger I was beginning to think nobody got it! I hope your sister learned not to karve her initials on møøse in the future!