Anyway...
You could always go to Eurodisney for the day
When I went it was the night after I went on a pub crawl with my mate and we woke up at 5am to catch the train there and got dragged hangover on the rides by my super-keen girlfriend. I still have nightmares about the Aerosmith ride.
Other cool things:
Day trip to Versailles (royal castle)
Catacombs (tunnels walled by human skulls, what could be more cool than that)
Les Puces (flea-market, I think the biggest in the world, not sure what days it's on though)
The Pantheon
Saint-Chapelle (small church, expensive to get in, but it's probably the most stunning church I've ever seen - highly recommended)
Musée d'Orsay (renowned modern art museum)
La Defense - this is the business district, it's worth checking out the Grand Arch, a giant skyscraper built in the shape of the Arc de Triomphe.
plenty more things, let me know what else interests you