Everyone enjoys something different about Europe. Here's a few ideas:

If you like countrysides, do the Cotswolds in England and I second Slovenia as a less busy, more affordable version of Austrian beauty.

If you like food, Italy is where it's at. And France.

If you like theater, you've got to spend time in London. They have so many different shows, it's more than just West End and the Globe, although both are great. Opera in Vienna can be really cheap, so do it!

If you like castles, Germany and England (although Slovenia has a really interesting one too in Predjama)

If you like Culture, probably a tour through Portugal/Spain/France for the wide variety in culture. Portugal is usually cheaper btw. Small towns are sometimes nice for culture.

If you like beaches, consider doing that at Venice. Rent a mobile home in one of their water parks on the beach.

If you like museums, London has some great ones and free.

If you like architecture, I liked Italy and England for the cathedrals. Do an evensong to see them for free.