My favorite place to bike is Parc naturel régional de la Haute Vallée de Chevreuse (French people aren't into the whole brevity thing). I can't speak to hiking but I have to assume there are nice walks to be had in the area because I frequently see people with hiking gear on. You can get there from Paris on the RER B: just take it all the way south to Saint-Rémy-lès-Chevreuse. Île-de-France is, by and large, quite flat, but Chevreuse is a nice valley so you can get some really pretty scenery.

EDIT: I'll add that last fall I was biking around there with my wife and we found some most righteous porcini mushrooms growing on the side of the road. It really is nice out there.

I should also add that nearer Paris is Forêt domaniale de Meudon, which is in the direction of Versailles. Nice forested area. I can't speak to the walking/hiking but I assume it is not bad (I've only been out that way on bike).