American with two weeks leftover in Europe this August after a month in Copenhagen, Denmark. Where should I go?

How well do you do really hot with little/no Aircon? If not we'll, head north.

A short flight to Lithuania, Latvia and/or Estonia?

Finland!

The Baltics would be the best to hit with your location. I hear good things about Oslo (not in a Baltic country but close by) and Riga but haven't been to either city. Estonia is good. Finland has a lot of great spots, like Helsinki, Tampere, Oulu, Inari (super cool cultural point to learn about the Sámi culture), Vaasa, etc. anything above the arctic circle like Inari or Alta, Norway is neat that time of year because it's near the longest day of the year and there's hiking and interest points.

If you find boarders fun, there's a lake (Kilpisjärvi on Finnish side) between Norway, Finland, and Sweden where you can walk out to a marker where the countries meet in the middle of a lake.

Otherwise northern France, like Honfleur, or Belgium?

You have been to spain, which part? You could do sevilla, jerez, malaga, ronda, Granada and tour along the beach and the mountains in a rental cabrio. Have you been in that regio ? That will be a superholiday.

Croatia

https://maps.app.goo.gl/aSkzXTrmtpfwPpnT9

Do you enjoy hiking in nature?

Helsinki

Sweden, Norway, Finland or Iceland

I hear Donetsk is lovely this time of year

Norway. Oslo, Flam, Voss, Bergen by train and fjord boat.

It would be helpful to know a little more about where you went in each country, as well as what you liked and disliked.

You should definitely get out of Northern Europe after spending all summer there. Maybe try Prague, Budapest, Granada, Ljubljana.