You don't say where home is? Your comfortable use of English suggests the USA, but "...flying across Europe" and "USA would be great ..." conflict with that assumption? ??
A wild stab here, just to get things started. If it were me and I had your budget and time, I'd look at Eastern Europe (I'm in the USA). I'd get several good guide books from the library, and pick places that intrigued me. Maybe start with Brno and head east and a little south. Non-capital cities and larger towns - maybe Gyor, Novi Sad, Belgrade, Timisoara, Craiova, Varna, and similar. Open-jaw air ticket that got me to one end of the route and returned me from the other. I checked a few of the cities, and in July it seems easy to find well-rated 3* hotels - single rooms about $50 per night (with breakfast). AirBNB places are probably similar. Summer weather is a little more bearable than SE Asia and one can drink the water. But the whole area is still wonderfully different and interesting.
Of course, if you wait until things cool off a bit, SE Asia would be a great choice also.