Buy a Mobilis ticket (zones 1-5) from the ticket machines, should cost around €15. From Chessy, central Paris is about 40 minutes on RER A. The round trip we did two weeks ago was :
1. Etoille station for Arc De Triomphe and Champs Elysees.
2. Trocadero station for the iconic view of Eiffel Tower from Musee de l'Homme.
3. Bir Hakeim station to Anvers (blue metro) for Sacre Coeur.
4. Denfert Rocherau station for the Ossuary (Catacombs).
5. Chatelet/Les Halles station for Louvre etc
6. Cite or St Denis station for Notre Dame, Palais de Justice, and Chapelle. Can also walk from Chatelet/Louvre
.
The leaflet for the Paris Visite card lists other stations near the touristy bits.