This is my first game jam and I made a game called bounce shot: https://itch.io/jam/gmtk-2019/rate/461622
It has one button control, there's one player, one enemy, you have one health, and it plays on one screen.
You need to kill the enemy by shooting your bullet at them, and bouncing it off the walls to get around obstacles. You get more points, the more times the bullet bounces before hitting the enemy.
I'd also recommend this game: https://itch.io/jam/gmtk-2019/rate/461470. Lots of fun, and a cool idea.