Where shall we go

It depends on where you've already been.

Let's see, looking at northerly countries...
Germany is a bit cheaper than the Netherlands in my experience. Poland & Baltic countries are even cheaper (no experience). Britain, while not cheap, currently benefits from historically good exchange rates vs. USD (even more so than Eurozone countries) - whether they remain so until June is crystal ball territory but it is worth a shot. Steer clear of Scandinavia & Finland.

This is so wide open, but since you mentioned outdoors, only have a week, and I believe there is a direct (seasonal) flight, perhaps check PHL to Dubrovnik.

Portugal? Economical, not hot, and about the shortest flights you can get out of Newark. You could fly into Lisbon and out of Porto or vice-versa, letting you see more than one city, but not have to backtrack for travel.

How about Poland? Very affordable especially since they're not on the euro, beautiful scenery and national parks from primeval forests to the wild Tatra mountains so good hiking, charming medieval towns and cities, fascinating 1000 year history, tasty food and beer. Weather should be nice in June too!

With one week you can easily combine Krakow with the nearby mountain spa town of Zakopane for some stunning hiking in the high Tatras National Park, just Google Morskie Oko to see what I mean:)

I think you can get a lot of this in one area - Cinque Terre. Wonderful fun little towns, lots of hiking and on your way there you can stop in to Munich and head to Genoa then to Cinque Terre.
2 days Munich
4 days Cinque Terre
1 day back to Munich

I would never pay the expensive airfare to fly to Europe for a week.

OK, you are probably still working, but can you manage two weeks?

In any event consider an Alaskan cruise or a trip to Montreal and Quebec City.

The options are almost endless! But just having come from Poland and Budapest, I would heartily second Carlos’s suggest for Krakow and the Tatras - or the whole week in Budapest. Both locations are very easy on the budget!

I agree with geovagriffith . One week doesn't give you much time especially when you take into consideration that you will have two days travel time going and coming back. There will be more travel time if you chose to move from one city to another while you are there. A lot of it depends on the price of your airfare. A few years ago my daughter and SIL scored flights to Madrid lat a price less than you can fly across country here. It was well worth it for them to take advantage of it. Of course they returned to work the day after their return flight. They did fine, but I'm afraid that I'm too old to even attempt that. That being said, I agree with Krakow. Poland is very economical, there is a lo t of history, the people are so friendly and there is so much to see. Private drivers are easily contracted and affordable. They can take you to Auschwitz-Birkenau and the salt mines. We loved Poland and would return in a heartbeat!

If you don't object to returning where you once were, (I don't ) those places in addition to new ones could make up your itinerary. How about Paris...always great for return visits...or Vienna? True, the Netherlands is more expensive than Germany.

It can be expected that going in the summer (my trips are in the summer) will be "crazy hot"...a matter you can lessen by where you choose to be. In France the "coolest" region in the summer is Brittany. Northern France is a few degrees cooler than it is in Paris.

Great Budget friendly for a week: Eastern Europe (20 to 30% less expensive than much of Western Europe "/" day trips or in transit stops "&" nights )

Croatia (Zagreb & Split & Dubrovnik)
Montenegro & Croatia (Dubrovnik/Perast & Budva/Stair Bar & Ulcinj)
Montenegro (Budva & Stari Bar/ Ulcinj & Tara Canyon - hiking)
Hungary (Budapest/Eger or Pecs & Szentendre)
Ukraine (Kyiv & Odesa or Lviv)

Nothing crazy hot compared to where you are traveling from - https://weatherspark.com/ :
June 30 temperatures along the coast (Dubrovnik) 70F to 83F
June 30 temperatures inland (Budapest) 59F to 79F
June 30 temperatures high country (Tara River Canyon) 47F to 66F
June 30 temperatures for Philadelphia 67F to 85F

I would never pay the expensive airfare to fly to Europe for a week.

I often pay less to fly to Europe than it would cost to get to Chicago or points further west form the Northeast.

Maybe Slovenia? Ljubljana is a lovely small city, with shops and restaurants along the river, and an interesting castle to explore. Then spend 2-3 nights at Lake Bled—-there are hiking trails, n exciting summer luge ( just ask my husband!), and you can rent bikes ( or borrow them from your lodging hosts and bike to Vintgar Gorge.

https://www.lonelyplanet.com/slovenia/julian-alps/bled/attractions/vintgar-gorge/a/poi-sig/1259467/360712

If you want more city time, Trieste is very close. For more unique opportunities, visit the Lipizzaner stud farm at Lipica. Or head to Bovec for rafting on the Soča River:

https://www.thesmoothescape.com/soca-river-rafting-in-slovenia/

Lots more ideas in Rick’s Slovenia guidebook.

The country is beautiful, uncrowded, and nearly everyone ( at least connected with the tourist industry) speaks excellent English. Prices are low compared to other alpine countries like Switzerland and Austria. Food is wonderful, a mix of Mediterranean and rustic/alpine.

Another vote for Slovenia! Or the Salzburg area.

I would also go to Slovenia. It is small enough to spend one week. Ljubljana, Postojna caves, lake bled. It is easy to drive around Slovenia also. Just make sure you rent hotels with parking. It isn’t too expensive either.

Given the time restraints one other suggestion might be to look at where you can get to efficiently and with the greatest enjoyment (good or no layovers, good departure time - maybe Friday after work, short flight time - avoid my favorite route through Istanbul, easy airports - rules out LHR and CDG)

Basically, leave ......... get in ............ have a blast ........... get out.

And if you can upgrade to Business on the way over and get some good sleep that will help.

Another enthusiastic vote for SLOVENIA!! We would go back in a heartbeat. Ljubljana is my favorite city in Europe. And as described up thread, so much to see & explore in this amazing country! https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjpjO2ckJP7AhX5AjQIHc-PAGkQFnoECAwQAQ&url=https%3A%2F%2Fwww.slovenia.info%2Fen&usg=AOvVaw09hTWRXA6clU-Mf3N0zoVe