Well. With no idea after 12 hours, I finally decided what to do:

(drumroll)

A platformer.

You play an owl, and start at the bottom of The Tower. Then you have to jump up all the way to the top of The Tower. What I got so far is:

• A blue screen, and an Owl made out of a few colored GL_TRIANGLE_FAN and a few GL_LINE_LOOP.

What I need to do next:

• Make vector-brick tiles for the tower.
• Draw a monster (kittens).
• Code (display of level geometry, platformer controls, collision detection, monster behavior)
• Draw a level (The Tower).

Optional, if I have time left:

• Animate owl and kitten.
• Some extra tiles besides the brick.
• Sound/Music