Something like this. Not springy but rigid. They sometimes come in a set of increasing heights so you can train your jumping height/endurance/speed/etc. A great thing to have if he's at all interested in track and field, basketball, skateboarding, parkour, or really any sport that involves jumping.
I used to have a blast working on my jumping when I was in high school. It's a crazy good workout and feels good to progress in I think.
I know you can get them at Walmarts and Dick's if those exist where you are but they're sold many places I'm sure.
your highness, the peasants are bored out of their minds
oh yeah? put em to work building some crazy shit before they start killing each other for a thrill