I'm a fan of pre-buying entrance tickets so I can forget about costs and not get a huge Visa bill when I get home. And there's some places - like Barcelona - where it was extremely helpful.
This summer, Budapest is on our itinerary. I don't see anything indicating long lines for most of our chosen sites - Hospital in the Rock, Matthias Church, Heroes Square, House of Terror and Jewish Museum. I AM pre-buying our Parliament tour tickets because I see that gets crowded.
Anything else I should be aware of try to pre-book?