What are some of the fastest starter concepts you can think of.  Something that would only take maybe 30 minutes to be playable and fun?

At any moment in time, there are probably 10 or more active game jams. (see : https://itch.io/jams) Most give you 1-3 days to complete a game, so we are often looking at what we could get done in that time.  But then I had a bug in my code today that pointed out a concept I could make playable from scratch in 30 minutes.  (this is in Unity) 

My starter: A spaceship is spinning. You have no control over its rotation (that was the bug), only its forward thrust.  through brief taps to fire the engine, you make it start to move in the direction it was pointing at the moment.  Just Rigidbody.AddForce. from a screen tap.  Then put a few objects to bounce off of any try to get the player from point a to point b.  Simple, less than 30 minutes, and the base game play is done.

What about other concepts like this, where a core playable mechanic is done in in < 30 minutes, so then you can just focus on adding new elements to the already fun video game.

Ultimately, I'm trying to come up with a bunch of these and share them out at game jams as starter point examples to help people (including myself) from getting out of big ideas.

Adding another: 
Space shooter, only needs 

 - left/right movement, about 4 lines of code.

 - space bar, fire projectile (projectile movement)

 - Asteroids randomly appearing from top and moving down.

 - Asteroid/projectile collision detection/removal.  

I know this can be done include cheap graphics generation in under 30 minutes.  I had built a slightly more polished AI Enemies in the game, done it in VB6, with star field, points, lives and game end, in 45 minutes 20 years ago.  and VB6 is not a good option for building games.