In reply to charlielow:
If you want to climb don't bother going to Paris, there's Karma in Font. I still don't understand why the FFME spent so much money to create a climbing gym in a great bouldering area, but it's still handy when it rains, though it's small since a third of it is for sport climbing, and another third only for the French climbing team athletes (so essentially we're paying for them, great huh?). It can be busy, last time I was there (one month ago maybe?) I think the Austrian Youth Climbing Team was there and it was very crowded. Better go during the day and avoid weekends.
Like someone suggested you could go for a walk, the blue Denecourt paths are absolutely lovely.
There's also the castle of Vaux-le-Vicomte North-East of Melun.