my husband and I spent £600 but we did buy a few items of clothing etc as well. It is v v expensive, the beer is extortionate, the wine not so bad but you only get a tiny bit!
Top things to do? Definately buy a guide like rough guide and have a browse through. Don't miss:
The Louvre - (but pick out the things you really want to see ie mona lisa, the venus de milo etc
EIffel Tower,
Galerie Lafayette (fabulous department store)
notre dame
arc de triumphe
you will only have time to do the basics.
Here's a tip - if you are there on a sunday lunchtime go to the Baron Rouge, a small wine bar (not like our wine bars) which serves wine by the glass and small plates of food. That is a flavour of real French city life. Its a way out from the centre though (get the metro) but worth it - theres a market there too.
Use the underground to get around, its very easy. One ticket = one trip, regardless of no of stops or changes.