Any other towns worth the visit by metro other than paris that is a safer bet
the trains won't be as frequent going outside Paris...so you won't have as much choice...just take the metro into Paris and find a nice cafe on the Champs Elysses for example and do some people watching...or sit in the Tuilleries...
I agree with Karen -- keep it simple! Find a nice cafe to hang out in the center of town, or stroll the Seine near the Louvre, Hotel de Ville. Enjoy.