If you're only interested in social experiences then there's all the obvious answers: girlfriends, team memberships, or even just how to discover and talk to other people my age with similar interests. Standing up for myself is hard. Simply talking to people is hard. How to filter the words I say so that people don't take offense to me expressing an opinion.

If you want to expand your question, then also: art, business, chemistry, college, even joining the military required a high school diploma.