Taking a cab from Charles de Gaulle into Paris is a bit pricey, probably around 40 euros. However, keep in mind you pay for convenience. You don't know when you are going to get back there again and you want to maximize your time. The bus thing is cheap but can be inconvenient to find and wait for when you first get there and things are unfamilar and you may be tired, etc. Once in Paris, you simply must use the metro. Don't be intimidated or scared (unless it's late at night LOL) as it is easy to use, extremely convenient, and clean! The #69 bus loop is awesome and goes to many major sights. You can just sit and ride on it as well for a nice tour of city. :)
Major sights I would recommend are: Eiffel Tower (bien sur!), Sainte-Chapelle (my personal fave!), The Louvre, Musee D'Orsay (a MUST see if you like Impressionist paintings), Notre Dame, Arc de Triomphe/Champs Elysee, and be sure to take the Seine boat ride at night. That's a great thing to do the first night to kind of set the tone. There are SO many museums there so be sure to think about what you are interested in ahead of time and find the appropriate museum and plan accordingly. For example, I LOVE Monet and mistakenly thought all Monet would be in the Orsay museum, but my favorite paintings, the waterlilies, were at the Orangerie which we didn't have time to go to! So, really get on the Internet and look for what you are interested in and have a game plan. You are going to have an amazing time--that place is magical! Have fun! Oh, and if you want to get out of the city for a day, you can take a day trip to Versailles, Giverny, or my personal favorite--the D-day beaches in Normandy. Extremely moving!