A lot of good ones were mentioned!

I guess my list would be:

1. Travel abroad and stay there for at least a month.

2. Take some time and try to understand opposing views. That doesn't mean you have to end up agreeing with them, but just try to understand what goes into them in an open-minded way.

3. I really like the suggestion above to live by yourself for some period of time. That offers an important learning experience!

4. I agree with the learn to swim and ride a bike.

5. Learn at least rudimentary music theory and explore different styles of music.

6. Read a lot, and different styles of literature. Read translated literature of a different country.

7. Have some basic understanding of your country's history, your local area's history, and world history. As a bonus, study a bit of geography, too.

8. Take the time to better understand, in an open-minded way, the issues faced by people with a disability. If you are not a person of color, take time to learn about the issues people of color face. Take time to better understand people with very different religious thoughts. Talk to them. Ask questions. Live among them for a while, if possible.