I have insomnia and am bored, so here is some stupid things I've done:
For starters, I used to really like jumping off of things. My mom has worked/still works at my old high school and, in their original gymnasium, there's a set of bleachers/stands/
Roman elective office holders that fold back flush into the wall, with some small gaps that were just small enough to climb. So I would often try to climb up the bleachers when no one was around and jump off at various heights. It went about a story up and do recall that one time I actually did jump from the very top. I also jumped from the balcony above it onto a pole vaulting mat below one night during a play practice that was going on on the stage across the other side without being caught (the main lights were off, of course).
Another bit of history: when my youngest sisters and I were little, we used to climb out onto the roof, an idea that might have been inspired by 3rd Rock from the Sun. One time, when I was alone, I decided to jump from the roof to the ground. This was a dicey situation, since even though this section of the roof was only a story up, it faced the driveway, and the only bit of grass was basically to the side of me. I had to angle myself just right to hit the grass without falling sideways due to the pitch of the roof, but I finally jumped and landed on my feet, just an inch or two away from the driveway.
Another time, when I was attending the high school I mentioned earlier, it was either during play practice or when I was helping set up the stage for a play, and there was a ladder there, about 10 or 15 feet, and I decided I would show off my jumping skills. I figured I had done this sort of thing plenty of times before, this should be no problem, but, I either misjudged the height or messed up my landing, because when I landed, I think I might have gotten the wind knocked out of me at first and I know I got a sort of pain in my feet that was spread out in a similar sort of manner to when they go numb, which lasted for several minutes. Fortunately, nothing got broken or sprained (I've never been seriously injured, other than one time when I was four and fell over while standing on some stacked egg crates and cut my top lip on a Duplo block and had to get stitches).
As for non-falling related stupidity, again around the time I was in high school, I used to climb inside some rolled-up wrestling mats that were stacked vertically inside the wrestling room. Now, most of the time, I was able to get in and out of them with no fuss. But there was this one time when I was climbing in one upside-down that I managed to get myself, not so much stuck as "in such a position that backing out again would require a lot of time and effort". It was after-school and my only ride home was my mom, one of the teachers there. When she was ready to leave, she noticed that I was nowhere to be found, so she sent people looking for me. Eventually, one of the teachers (I believe it was one of the wrestling coaches/the biology teacher) found me and pulled me out, or rather just tipped the thing over so I could climb out. After that I had a reputation as the guy who got stuck in the wrestling mats, which actually worked out positively for me, because my high school was relatively small (compared to public high schools) and people there actually appreciated weirdness. Which reminds me, at that time I had a thing were I wouldn't wear shoes, so every day, all throughout high school, I wore sandals, or sandals and socks in the winter time (I wore boots when it was snowy or raining and changed when I got there). Also, on a lark, I sometimes carried a dime in one of my socks.
Similar strange impulses have also caused me to do such things as sleep on the floor with my feet in a plastic container of water; strip naked, stand on my clothes and pour a jug of water over myself; and going to sleep with silly putty in my hair. In the last case, I tried to cut it out, but it was later explained to my that it could just be shampooed out. So I was left with a chuck of my hair gone, and on that day my mother and I were supposed to be extras in a show that was sort of a worse knock-off of Seventh Heaven being produced by someone from our church.
For things I believed, I didn't know exactly what my dad did until I was probably in my teens or so, so when I was little and they asked us what our parents did, I said my dad was a hunter because all I knew about him was that he went hunting sometimes. I also thought we were Polish because my dad said my mom was Polish, which is an insult I still don't understand (and haven't heard in a long time since he stopped using it quite a while ago). There's probably others, but it's harder to remember incidents of belief rather than incidents of action, and I'm not even that great at remembering the latter.
EDIT: Oh, just remembered. When I was little, I was imagining what the Lion King would be like if there were people in it instead of animals. And for some reason, I imagined everyone as being naked (I guess because animals don't wear clothes?) This was of course before I learned what vagina looks like, so, when I imagined the girls, I imagined them as having penises, just slightly different than the boys (I don't remember how.)