Where's the stopover?
If it's at Gare du Nord, you've got loads of time. (There are plenty of attractions quickly accessible by RER or Metro services).
If it's at Charles de Gaulle airport, it takes about 40 minutes (on RER Line B) into central Paris.
If it's at Orly airport, it takes 8 minutes on the 'Orlyval' (driverless train) to get to Antony station where you pick up RER Line B for the 25 minute journey into central Paris.
http://www.aeroportsdeparis.fr/Adp/en-GB/Passa gers/Arrivees/QuitterAeroport/EnTransportEnCom mun/EnTransportsEnCommun.htm
If it's at Beauvais airport, you're about 55 miles from central Paris and there wouldn't be time to see anything.
Given that RER Line B serves both of the major airports (as well as Gare du Nord), it might make sense to head for St Michel Notre Dame station (which is on LIne B). As the name suggests, this is right by
Notre Dame cathedral, which is worth a visit. The surrounding area is generally attractive and includes the
Latin Quarter (where there are loads of good, but really cheap, restaurants).
St Michel-Note Dame station is also the point where RER Line B meets Line C. It's only a short journey along LIne C to get to the
Mus�e d'Orsay (which, as Rosetta indicates, is a superb museum). However, it's only another few minutes along the line to Champ de Mars-Tour Eiffel station, alongside the
Eiffel Tower.
Chris