Invitation to Inanity
Take a thin pen with a cap. Uncap it and place pen and cap in the palm of your hand. Now, shut your eyes and recap the pen, without the aid of your other hand or any other object or surface, and without marking yourself.
With my pen (a Pilot Precise V5, cheap and common), hand size, and dexterity level, this has proved surprisingly fun and challenging.
Play at your own risk, obviously. As a lefty, my hand is usually ink-smudged just from writing, so it's no big deal, but watch out for the clothes.