Castles / Les Châteaux
The must see castle near Paris is Chateau de Versailles, but if you do have the time, Fontainebleau and Malmaison are worth a visit as well.
For most castles you can either do an independent visit and get an audio guide or reserve a group or guided tour in which case you may need to make a reservation long in advance. The guided tour can be arranged by the museums or castles directly or with other tour companies. You can easily book your guided tour with La Conciergerie online.