What area in Paris is best to stay in that is easy to access from DeGaulle and then make train stations for Spain, but is within walking of central sites?
It's easy enough to get from the airport to any place in the city. It's easy enough to get from any place in the city to the train stations. If you're going toward San Sebastian, you'd use Gare Montparnasse. If toward Barcelona or Madrid, either Lyon or Austerlitz. (These two are a five-minute walk apart.) Austerlitz and Lyon are closer to Kilimeter Zero (Notre Dame), but it's still about a mile and a half hoof. Montparnasse is about two miles away.
What Ed said. If you'll give us a budget range, we can suggest some hotels. The Metro and RER are fabulous and will get you anywhere you want to go, including train stations. My experience is that the closer to the river you are in the city center, the more sites you can walk to.
I'm travelling with my high school aged son, so we are looking for clean and cheap, not fancy. I'm too old for the youth hostels :(
Greg, Hotel Diana, is clean, cheap and VERY central . You will be able to walk around the Latin Quarter, over to Notre Dame and Louvre, and its only a few blocks from the St Michel RER line stop that will bring you in from the airport.
The metro system is very easy to use and catching a metro to the station you need will be easy( but where in Spain are you heading, I assume you have already purchased your train ticket, so which station do you need to be at to go to Spain?) Another choice is Hotel Eugenie, central , clean, a bit more money but not much, and both hotels have nice staff and are near all sorts of cafes and shops, both within easy walk to Seine etc.
We can be more helpful if you give us an actual price range, in Euros. "Cheap" means different things to different people. And yes, since you are only taking one trip in from the airport and one trip out to a train station, don't choose your hotel based on this. As stated above, hotels closer to the Seine are an easier walk to sights, but they are also more expensive for that reason. And no single location is "central" to everything you will want to see. If saving money is a prime objective, look at hotels further from the Seine; these can still be in interesting and nice neighborhoods. In addition to Rick Steves' recommendations, have a look at EuroCheapo's Paris Listings. But be quick, because these places are not a secret, and fill up fast. I stay at the Grand Hotel Des Balcons, between the Odeon Theatre and the Odeon Metro stop, in the 6th. The decor in the rooms is basic (the lobby is fancier), the location is great, and the price is right - for me.
My sister and I stayed at the Hotel Cluny Sorbonne, which is a 10-minute walk from the Seine. We had a room for 2 people for 100 euros per night. The room had its own bathroom and an optional breakfast, which we didn't purchase because we bought crepes every morning for 3 or 4 euros.
www.parisbytrain.com gives simple advice on using all the public transit systems including the regional train from CdG into the city. Paris is easy to get around and access to the Metro subway is more important in choosing a hotel than the actual city area. I like to be near a station where more than one line stops, increasing the choice of routes to city attractions.