There's a discussion on Reddit where people are sharing the very simple things that other adults seem to be fine at doing, but they're embarrassed they can't do.
1. Swallowing pills.
2. Remembering directions.
3. Tying a balloon.
4. Confrontation.
5. Dancing.
6. Remembering people's names the second after they introduce themselves.
7. Whistling, especially with the two-finger method.
8. Shuffling a deck of cards.
9. Tying a tie
10. Winking.
(Reddit)