I agree that you need to think cities not countries. Budapest and Vienna are comfortably close. Without flying you could visit, another city like Prague; Munich; Krakow or Venice if you are willing to take a night train. Or you could visit someplace smaller like Saltzb7urg, Cesky Krumlov, Eger, Pecs, Innsbrook, etc. If you fly within Europe you could go most anywhere.
Vienna is not my favorite city to be in. It's not a grat city to wonder tjough it has many huge and beautiful buildings. But it is packed with palaces and world class art museums. You could spend a week there looking at art and other run out of things to see.
Prague is simply a breath taking place to be. It is crowded, though maybe not so much in Aprril. There is art, particularly Mucha's art nouveau, but it's not an art destination. The multisite Jewish Museum and the castle complex are both well worth seeking. The main square is one of the best in Europe. And it's still affordable.
Munich has more art and a rebuilt palace, not to mentionthe BMW plant and beer.
Salzburg is is pretty, but while big on atmosphere has less sight to visit than Prague. Ditto Cesky Krumlov, though for whatever reason we liked it better.
I haven't been to Budapest or Krakow.
If it were me, I'd spend all of mry time in Budapest and Vienna, with perhaps an overnight in someplace much smaller.