Cracked has a bit on things that explode that you might not expect to. One of the items listed is the seed pod of the sandbox tree;
The seed pods are about 2-3 inches in diameter and reminded me of a radial aircraft engine, but with a hole in the center
This one still has a stem attached. When I was in Viet-Nam, the kids there used this seed pod, which is woody and very hard, as key chain fobs. With limited language skills on both sides of the transaction, and an easy way to have some fun at the G.I.s expense, they provided me with a large one, which I brought back with me. It was sitting on my step-fathers mantle when it germinated, throwing the ribs a good 30 feet.
The Stanford botany department now has a full sized hand grenade tree of their very own.