Small prototypes- Every mechanic you can think of, from swinging, hookshoty movement, to cars,boats, planes, first person movement, third person movement.
RTS prototype. Pathfinding. Platformer and topview style games. Picking up coins and picking up weapons.
Anything you could later turn into a 'toolkit' or full fledged game.
Things like simple level editing where you place and remove prefabs by clicking.
I could go on all day with ideas. Flick your fingers through the mobile app stores. Clone anything at random and you'll end up with a good video.
My only suggestion is that- while we like how 'live' twitch can be... Don't wing it. Getting stuck on a problem for a long time can be the death of a good tutorial.