Tiny Green Men

This planet looks perfectly ordinary from orbit, though if you have good sensors and/or a good science officer, you'll probably be able to spot the dirt huts that suggest there are semi-civilized aliens. The tiny green men (okay, "0.5m high humanoids") have a problem: they love mushrooms, and the best mushrooms grow in caves, but there's a monster in the cave.

This is a perfect quest for Captain Kirk: go into the cave, kill the alien, declare peace and prosperity for all. This is a "mostly harmless" world because there's only one monster, and he's not all that tough. Once you've killed the monster, the tiny green men will be ecstatic, and the first one you talk to will reward you with several pieces of alien art (which, of course, is nigh-worthless unless you've found the Independent settlement).