When I'm Bored, I make shit. I've made quite a few rube goldberg machines, my favorite one was a machine that started by putting a weight in a cup near my bed, and after like 40 seconds of madness, it turned off my light. Best way to go to sleep ever.
I also rather enjoy making small weapons (read: things that shoot stuff across my room) from the things on my desk. It's a good game, actually: sit down at your desk, and, confining yourself to the materials on your desk, se how far you can launch a Lego man's head, or just a bb.
Hands Down my best invention was when I was 11, I would cut off the neck of a ballon, and stretch it around an empty scotch tape roll. Then I'd wrap a bit of electrical tape around it to hold the ballon in place, and use it to shoot dried berries(for a painful hit) or fresh red berries(for a painful and bloodied/looking mark). My friends and I had wars that lasted hours, it was like an airsoft war but with unlimited ammo because the bushes were everywhere.
Everybody made their own "guns" at first, but my design won out in the end as people decided that my hits looked more painful than theirs.