[Dreaming.]

I've heard about some fab hostels in Paris... Er, give me a minute.
I don't know if hostels are your thing, but they're always pretty fun and full of life (and cheap!).

As for places to see; if you're going overnight, maybe try to get a two day metro pass. It just makes travel so much easier! Try to go up the Arch du Triomphe if you can; it's a lot of stairs, but it's so worth it - and you don't get anywhere near the queues you get with the Eiffel Tower. The Latin Quarter has always been pretty fun and full of life when I've been. I've also always preferred the Musee D'Orsay to the Louvre when I've been, but I'm not sure if that's just me :P Montmartre is pretty fab to wander around, but watch out for creepy people tying bracelets around your wrists. How are you getting there? If you haven't sorted that yet, I totes recommend the Eurostar as it just gets you right into the city centre (from LDN city centre) in under two hours, which totally beats check-in queues. But ignore me if you've already sorted that. :)

Both the Rough Guide and Lonely Planet do pretty amazing budget guides to various cities; maybe it'd be worth checking them out?

Hope you have an amazing time! I love Paris; I've been four or five times at least now, and it's just so vibrant and full of all sorts of amazingness :)

ETA: Hostelwise, I remember hearing good things about the Peace and Love Hostel, and St. Christophers (although that's pretty much guaranteed to be okish, since it's St. Chris') - and I think we've stayed in Les Trois Poissons and Grand Hotels des Gobelins before, both of which I remember were decent enough :)

Have a great trip!