I got stuck in Paris while waiting for my interrail pass to arrive, but it wasn't really a problem because there is so much to do here. A lot of the famous sights are free if you're from Europe and under 26, even things like Versailles and the Louvre. I couldn't get the hang of the metro, and it gets expensive quickly if you use it often. Luckily most of the sights are close together - except the eiffel tower and Versailles. I didn't go out so I don't know about the nightlife.
Insider Tip:If you're under 26 and part of the EU, you don't actually need a ticket for the louvre or versailles. You can just show your ID and get in. If you're using the metro, buy a book of tickets. And if all else fails, just go to a park; there are a lot of them and they're almost all beautifully done. Oh and Shakespeare and Company has a library, where you can sit and read if it all gets a bit hectic. :)