Only half a day in Paris

You'd be crazy not to do that. What do you want to see? Off hand, I'd take the RER to St Michel-Notre Dame, walk along the Seine, go to Notre Dame, St Chapelle, have a nice dinner, see if inspiration strikes. Last RER back to CDG is about midnight.
Edit. Get a Mobilis Zone 1-5 day pass for 16.10 euro. That will cover you to and from the airport, plus any Metro or bus journeys in Paris while you are there.

Absolutely do it! A 1/2 day in Paris is better than no day in Paris! Be careful - you will want to return! Be sure and catch the Eiffel Tower all lit up. Do a little research and have a nice dinner somewhere. You would even have time to do a day or night bike tour of the city which would give you a great overview of the main sights.

It is a shame not to have more time but take what you have.

You should be realistic about the amount of time you actually have... It seems that you arrive in Paris at noon. That is at your hotel at noon, all checked in and ready to run out the door to the RER?

Or is that when your train arrives or your plane touches down from wherever? So that by the time you have checked in, dumped luggage, changed clothes, etc., it is what time?

Thanks, everyone! Our flight from Prague gets in at 11 am, so we'd probably be able to be onto the RER by 2 pm at the latest. That would give us 7 or 8 hours to sightsee.

Definitely go into the city. What else would you do, hang around CDG all afternoon and evening? ;)

Unlike some of those who ask about seeing Paris (or London) with a few hours between flights, you have no risk of missing your outbound flight as long as you get back to your hotel the night before. RER into the city and make the most of it. You might consider one of the hop-on-hop-off bus services to maximize your exposure to the city and still let you visit a few places in more depth. And/or take a Seine cruise, evening would be great (but dine on shore). Drag yourselves back to the hotel, tired but happy, and set the alarm to give you plenty of time before the flight.

I agree with Sam and would add visiting the Eiffel Tower at night and taking the one hour r/t Seine cruise that leaves a block from the Tower. The city is magic at night from the Seine. The hop on hop off gives you a good overview and stops at all the best sights.

Eiffel tower at night, arc de triomphe (if you are in good shape you can climbed the many stairs to the top and look out all over the city, including a commanding view of the Eiffel Tower) and, even if I had only 20 minutes I would opt for the Musee d'Orsay to feast my eyes on at least a few impressionist paintings (night hours to avoid wasting time in line?).

It's at least nice that you have a half-day with a bed waiting at the end, instead of a half-day with looming airplane departure deadline. You don't necessarily need to go into any museums or sites, but Notre Dame Cathedral is free to pop in, if the line is not daunting. You would get a good view of the city by walking, taxi, public bus, and/or taking a river boat. You can download Rick's Historic Paris Walk audio tour to your mobile device (matches the route in Paris book). Your hotel should provide a city map when you check in.

More people would like having the luxury problem you have. I too think an hop on hop off bus tour is a good way to have an overal first impression. But if you make effective use like Sam already noticed of your Mobilis 1-5 Zone daycard you can easily get to most of the main attractions. Travelling with the metro goes very quick and as soon familiar with the system very easy.

With RER-B from CDG you can indeed get out at Saint-Michel Notre Dame and after visiting the Notre Dame you can use metro (yellow) line 1 as a kind of a baseline for most other well-known sites. Within easy walking reach of it are: Hôtel de Ville, Centre Pompidou, Louvre Museum , Place Vendome/Rue St-Honoré, Orsay Museum, Place de la Concorde, Les Invalides, Champs-Élysées and Arc de Triomphe, from there to La Defence with RER and/or also with metro (light green) line 6 to Trocadéro for that famous view of the Eiffel Tower.
From Arc de Triomphe runs metro (blue) line 2 to the Moulin Rouge (station Blanche) and Montmartre/Sacré Coeur (station Anvers). Nice place I think for a last view of the illuminated city in the evening. From there Gare du Nord is easy to get to with (magenta) line 4 for the RER-B back to CDG. Be aware to keep your Mobilis card till the last station for getting out.

Check the B Line ending times and shoot for a couple before the last departure so you don't wind up having to take a taxi back to the airport.