I assume you mean IRL activities?

• Lock picking,
• knitting/crocheting,
• learning to roll a coin over your knuckles,
• working with beads or chainmail,
• definitely piano but other instruments can also help,
• and also typing. Home-row, no-looking, speed typing, is I guess what I mean.

For Ambidexterity, brush your teeth with your non dominant hand.

Guitar and magic tricks.

Ideally both at the same time.

Guitar, piano.

That knife game where you stab the table between your fingers as fast as you can.

Playing a piano can do this because the velocity of each keypress is a big part of the expression, and it’s a scalar value meaning infinite degrees of possible variation.

Piano playing and touch typing come to mind

Air guitar

Have a look at pen spinning

Boxing