1. See how long you can hold your breath. Optional equipment: stopwatch, or watch with a second hand, or some other way of measuring seconds. Alternative: just pretend to have a stopwatch, because the trick here is to fool other people, especially noisy children, into having a breath-holding contest. We did this a lot at my house when I was a kid, usually at my father’s instigation, and I only figured it out recently.
2. See how long you can hold your hand in icy salt water. This used to be the funnest part of making hand-cranked ice cream when I was a kid. My brother always won, because he was insane*. Necessary equipment: bukkit, water, ice, salt.
3. Jump out of a hiding place and scare people. My wife and Gamma did this to me a few days ago, and they laughed so hard!
4. Make rubber-band guns out of junk and pieces of inner tubes, and have wars. (My mother still fondly recalls watching the big kids do this back in the Depression)
5. Dig a hole.** Necessary equipment: shovel, dirt.
6. Meditate. Optional equipment: cats.
7. Fly a jumbo jet. I bet that rocks. Drawback: need a jumbo jet. Advantage: if you have a pilot’s license, they’ll actually pay you to do this.
8. Go into a Banana Republic and ask to use their telephone. We tried this in Seattle a few years ago, this freaks them out for some reason. Maybe they think you’re poor. When they refuse, ask to at least see their phone book.
9. Make a list of all the things you’re doing wrong, and correct them.
10. Find out where the nursing college dormitory is, and when shower time is, and sit in a tree outside (this works better in summer than in winter, due to foliage considerations). This is only free if you already have binoculars.

*He no longer does this, having discovered spicy chili pepper eating contests.

**Works for me.