Nice simple Lunar Lander game for the Nintendo DS. Relieve the 60s and early 70s by flying around in your Apollo lunar lander module.Tested and operational on R4DS. Uses DLDI for high scores (optionally).
Or when your player dies, you can say in code, if enemy exists, do nothing, but if enemy does not exist, then create enemy at certain spot. (This would be a pain tho for lots of emeies)