In reply to dsharaf:
I think that it is possible to get off at Bois-Le-Roi station and walk to Rocher Canon in half an hour or less?
Would definitely get the "top 25" map for Fontainebleau, as it is easy to get properly lost in the forest.
Take all the water and food you will need, and there are no toilets either!
Personally I would take a pad, but there are shorter problems with flat/sandy landings depending on how confident you are (old school?!). You may be able to hire one but without a car it could be tricky?
Have fun, Font is the best!