I don't have much experience with this sort of project, but I have seen and used foam myself. Are you talking about making a bench and covering the sitting surface with foam? If so, you could get a piece of foam and cut it to the size you need with an electric carving knife (like for a turkey). Then you use spray adhesive to tack it to the wood surface. But then you would need to cover that with fabric somehow. You could also spray adhesive the fabric and tack it to the underside to pull it tight before you secured it on its base.
If you are talking about a vertical surface that would be like a contination of the walls, only in wood, I agree that making it a chalkboard/magnet board/bulletin board would be the easiest and most fun solution. All those paints are available at craft stores and hardware stores, and you can get cork board in a roll and roll it out and spray adhesive it to the surface. They you can get pieces of molding to create frames around the areas and finish the rough edges.