I'm finding this difficult because what I remember most are the things I did that I don't regret. I wasn't a 'crazy wild' teenager type although I did have a few adventures. As evidence of this, when I was around age 22 my older sister (15 years older!) said to me, "What ever happened to you? You used to be SUCH a good little boy and now you've become SUCH a problem *adult*!" LOL… She was dead serious and I laughed right in her face, YAY!!!
But, no I didn't do a lot that I regret as a teenager, sad to say. I did fall out of a tree and broke my left arm… but I can't say I regret that, actually. So far as I can figure out in retrospect, the anesthesiologist shot me up with ketamine, leading to my first psychedelic like experience. Seriously, it tripped me totally out. As I was counting backward from 100 I saw this spiral galaxy spin out of my heart chakara, and I didn't even know I had one! When I woke up the next day I felt like *Superman*… that shit was AMAZING.
I suppose the on thing I regret most is being a bit of a dick to one of my younger nephews. There were several near my age (my brother and sisters were much older than I, and they tended to breed young), but one in particular that I picked on a lot. He was 4 years younger and we often got into spats. I didn't seriously dislike him or anything but I knew how to get under his skin.
Well, later on, when he's a freshman in college, he confronted me about my abusive behavior (never physical). I really got it, I'd been an asshole about a lot of things, and so I apologized. We actually began to become good friends at that point. He, too, was gay and died fairly early on during the AIDS plague. So… I regret having been a dick to him but I'm glad we got it cleared up, too.