denim is great for boys, too--it wears well, and gets softer with each wash. and if you ahve acces to worn out jeans, you can get squares from the lower legs, and use the patch pockets from the back as squares, too. it's fun to "hide" the "hot wheels" in them--or stuff a paperback book into one.
work bandanas would be nice as a top, or applique, too. and there are harry potter prints and panels out there, as well.