Tuna w/ mayo (kewpie if you have it), cooked baby shrimp, imitation krab (formed whitefish), plain cream cheese, or just straight rice are some of the ones I've made and liked. More complex ones like the same ingredients as popular American sushi rolls work also as onigiri.

Almost forgot breakfast style ones with a bit of egg, er tamagoyaki, and diced ham, bacon, or sausage.