PHLFlyer's post is full of good advice. I'll add that the train station is right below the Terminal 2 building at CDG, so it's super-convenient. Unless the situation has changed in the last few moths, it's impossible to use a US credit card in the ticket machines at the RER station at CDG, so if you're US-based you may have to stand in line to buy your ticket from an attendant.

I've stayed many times at the Agora St-Germain (the last hotel PHL Flyer mentioned). It's lovely. The same people own the Abbatiale, so I'd trust that to be good also..

That really is the best neighborhood for a one-day Paris experience. Aside from Notre Dame (which you'll see as soon as you come up from the Metro station), the Sainte-Chappelle, the Louvre and the Orsay, there's the amazingly good ice cream from Berthillon on the Ile St-Louis. I especially like cacao amer (bitter cocoa) and agenaise, which is flavored with armagnac and prunes.

For dinner in that area, I still love Brasserie Balzar after many years of visits. They make a wonderful roast chicken with frites, and equally wonderful steak tartare. Despite being very popular with tourists, it still fills up with locals a bit later in the evening.

If you want to eat at one of the major. pricey temples of gastronomy, book well in advance--and see if they have a prix fixe lunch. That can often be had for a fraction of the cost of dinner.

If you'll be in Paris on a Sunday, definitely walk along the quays of the Seine, which are closed to car traffic for much of the day. It's fun.

If you like very mainstream classical music (lots of Vivaldi), I'd suggest taking in a concert at the Sainte-Chappelle. The concerts are only about an hour long, so you can fit one in before a late-ish dinner or after an early dinner. The building is an astonishing gothic masterpiece, and the acoustics are super. In summer, the sun will still be up for the start of the early concerts, so you'll be able to experience the windows properly.

Finally, when walking around Paris I always stop to read the many plaques all over the city (but particularly plentiful around Notre Dame) which read "ici est tombé pour la patie..." They mark the spots where resistance fighters were shot by the Nazis during the liberation of Paris. To stroll through that beautiful, civilized city and imagine it as the sight of deadly street battles is a profoundly moving experience.