As a European, I highly recommend the Czech Republic. Specifically the city of Prague. You can live like a king (queen) for pretty damn cheap, as its in Eastern Europe (which by and large is pretty shitty), but it's still really nice there. It's the best of both worlds really. Also, you can get drunk off of great beer - namely Pilsner Urquell, or Staropramen if you prefer, for pennies.
Don't go to Paris, it's a shit hole. If you have to go to France, go to Nice or Toulouse. Or Carcassonne if you want to see a bad ass castle-town.
Sweden and Switzerland are the two countries I lived in the longest, but both countries are ridiculously expensive. I have many fun memories, but as a tourist I'd say don't spend more than 2 days in either, or you'll go broke before you know it.
I'd say hit up Berlin for the night life. I don't know if you like house music, but they have the best nightclub scene in the world IMO. Minimal House is huuuge there, and it's so much fun.
Also Italy is great in the summer. Go to Venice for the full experience. Avoid the big cities for the most part. Smaller ones are always more fun. Though, I have heard good things about Rome, but I've never been so I can't say much about it.