Mostly everything that teenagers do. Hang out, date, make friends, learn to socialize and not be constantly anxious around other people. Things like that, things that involve socializing. I tried to make up for it in college too by partying and doing plenty of irresponsible shit (because I felt entitled to having the teenage years I was robbed of). I had no problems graduating, but no one else who tried to keep up actually graduated. I'd say learning to socialize and be around people is more valuable than anything else I did learn as a teenager, by far.