My wife and I went to Paris over the summer, it was amazing! I highly recommend staying the extra time (Assuming you don't have any 'hair on fire' debt emergencies, etc). Especially if you think you might not get back there for a while.
We stayed at the Hotel Choiseul Opera
http://www.hotelchoiseuloperaparis.com/ which was very nice, had an optional continental breakfast which was very good, and they have a friendly cat that lives in the hotel. It is near the Opera metro station which has three lines running through it. It is also very close to the pickup point for the Roissybus which will get you back to CDG. There is also a velib station in front of the hotel if you feel like doing some winter biking.
You could also check airbnb or vrbo for places to stay. That way you can get a place with a refrigerator, grab some fruit / cheese / croissants from a deli and have breakfast in your room and possibly take some food for the road. You can also get some street food like a crepe (ham and cheese or chocolate and banana) for a few euro.
Food prices from cafe's look expensive, but they include tax and tip (I usually left 1 or 2 euro tip anyways) so it's only marginally more expensive than usa prices. Soda and alcohol will be very expensive (3 euro for a can of coke at a restaurant), so ask for 'carafe d'eau' (regular tap water) to drink if you are eating at a restaurant.
Of course you can go to the Eiffel Tower and the Louvre, that probably goes without saying.
I particularly enjoyed the Musée de l'Orangerie which has Monet's Water Lillies (highly recommended!)
The Set in Paris movie tour was great too (but a bit pricey)
http://www.setinparis.com/For money... unless you have a 'chip and pin' credit card, which is rare in the USA (99.9% of banks that have chip cards give you a 'chip and signature' card, which is useless) you cannot use your credit card at automated metro stations / velib stations / gas stations etc. Fortunately your debit/atm card should work in Paris to get cash, and your regular mag-stripe cc will work at restaurants and anywhere that is not an automated kiosk. The ATMs did not charge a fee (but your bank back at home might).
Lastly, as an FYI, when you are around 'touristy' places, if anyone asks you 'do you speak english' and 'will you sign a petition for sick kids' it is a scam, just say no and walk away, they are just looking for money and it's not for sick kids =) I was caught a bit off guard when a group of teenage girls were roaming around the Eiffel Tower doing this.
Have a good trip!