When I have a few minutes I want to fill with something mindless, I like going round levels in freeze mode and placing a rocket in front of every monster on the level. I try to make it so I place enough rockets to kill each enemy but I also try not be wasteful. It's also quite tricky because too close and the rocket explodes on the frozen enemy and damages you even though it freezes immediately afterwards. Anyway, once all my rockets are placed, I go back to the starting point and unfreeze. Then I check the kill counter to see if I got them all.
Of course, you can do it with other projectiles, but I like using the rockets myself.