Here are some mockups I did for a game design last year.
The general premise is a 2d physics game with multiple single-screen levels. When the level starts, you have a few seconds to rearrange whatever items/furniture/planks of wood you find lying around and place your hero somewhere on the screen. After the initial timer has run out, the pipe opens and hundreds or thousands of tiny zombies or robots (or robot zombies) pour into the level. You have to avoid being touched by them for 30 seconds, after which the level is cleared and you're on to the next level. The difficulty is in being able to quickly rearrange the furniture and choose your safe spot to prolong the time before they get you. As they are able to eat through most things, they would eventually get through.
There is the possibility for tweaks to the difficulty setting by changing the amount of available furniture, whether the protagonist can move mid-level, how many zombies there are, how small they are and so on.