How about, instead of a present for dad, make it a present for the special man in the child's life? This way they can make it for any man they are close to.
I like the rock idea. You could take the boys on a nature hunt to find the rocks. Have the boys help you think of short sayings. Some suggestions "You Rock" or "Rock On".
HTH,
Judy

p.s. They could also make more than one other people.