Bobbing for apples is a good idea, but the food coloring would color their faces. How about if we fill a large pale with some kind of nasty, gooey stuff (maybe ketsup/tomato sause mixed with hair gel or something like that) and fill the bottom of the pale with tickets that have a prize written on the back. If we make it scary enough, it'll be fun to watch them try it. If they get the courage to pull a ticket out, they'll win the prize.