I second 'An Hour from Paris'. She focuses on less known towns e.g. not CHartes, or Reims or Rouen or Giverny or Auvers sur Oise, all of which are great daytrips from Paris. Some we have gone to mentioned in her book include Rambouillet and the Presidential Palace/chateau there, Senlis, Moret sur Loing, Crecy la Chapelle, Seaux, Ecouen, Conflans Ste. Honorine as well as the guinguettes on the Marne and the Seine. You can see a few of our more recent choices here: https://janettravels.wordpress.com/category/day-trips-from-paris/
These are all very easy to do and take an hour or less; if you have a Navigo Decouverte travel is covered to any place in the Ile de France including all of these places. Another one to add to your list is St. Denis basilica located on the Paris metro just over the city borders. Oldest gothic church in Europe and the site of the tombs of the Kings of France. Their bodies were trashed during the revolution and may or may not be returned to a mass grave here, but the tombs are magnificent.
Note that small towns with a largely tourist economy tend to shut down tight on Mondays so don't pick that as your day to visit.