Have really good sex and really bad sex at least once each. It really helps you better know what you like/dislike/want/need and how to better please/communicate with partners.
Cut a toxic person out of your life at least once. If you've never had anyone toxic in your life (or never removed them), it is really difficult to be aware of toxic behaviors and how they can affect people. Also, it helps you understand just how hard it can be to actually remove those toxic influences.
Help someone through a very dark time. It can be draining, but it is rewarding and will truly help you understand that you can seek help and that you can provide help.

Spend some time next to death. Volunteer for hospice. It will make every day afterward more appreciated, vivid, and intentional. Material things will not matter as much. Relaionships with friends and family will be more important because you will see how short life really is. You will also come to realize the only immortatlity currently available to us is the enduring memories of ourselves we create in the minds of those who remain after we die.

[americanhospice.org]

Theatre.

There's a lot you can learn about yourself by getting out on stage in front of your peers and strangers and performing a show. Nothing will teach you humility better and help you understand the plight of others like making yourself vulnerable on such a large scale. I still think everyone should have to take at least one theatre class early in life but ONLY if they get to perform.

Death.

Die ... You only get to do it once though .

You haven't really lived until you've had a mortal enemy.