Twiddling a coin or pen around your fingers.
Origami and/or balloon animals!
That paddle-ball-on-elastic-string thing. I’m fairly coordinated in most respects but I’ve never been able to do this. Of course, I try for about 30 seconds once every few years or so, so I shouldn’t bitch about it.
Learn to whistle with your tongue against the roof of your mouth rather than pursing your lips. Then confuse people by smiling as you whistle. It drove my kid nuts for years, then she worked out the trick and now dives me nuts by doing it all the time.
Similar to the paddle and ball, holding the string and repeatedly punching a balloon so it keeps returning like a punching bag.