If you plane to stay 3 or more days, buy a mass transit pass called "carte orange hebdomadaire zones 1-5". It costs 27,65 Euros for a weekly coupon which gives you unlimited travel by bus, metro and some rail lines in Paris and surrounding vicinity. It is also valid for tranportation to both airports. You need a photo.

 

A weekly coupon for Saint-Quentin - La Défense costs 15 Euros. It does not allow travelling within Paris.

 

For more information on fare, services and time schedules, have a look at the RATP or SNCF web pages. They both have an English version.