We hired a clown for my daughters 3rd birthday Breeze. We had the same one for her first birthday too and it was super hit. She did a few magic tricks, face painting, balloon toys and in general did all the funny stuff the clowns do. She got her own supplies. She didnt need a lot of space either. Mind you though, all the kids who came were under 7 years old. Dont know if a 12 year old would be interested in a clown.

If your son is into any characters, see if you can rent someone.