Their favorites are bugs and worms (flightless fruit flies, crickets, red wigglers, mealworms, waxworms, superworms). Second favorite is seeds (we've done sesame, flax, and BOSS). Then they've gotten a whole host of sprouts, chopped/diced veggies, some fruits, grains, fodder (mixed grain and legume). They've been getting clumps of weeds lately (dandelions are a favorite).
The only things that mine wouldn't eat so far was warm oatmeal and canned green beans w/ roasted garlic. Since I gave them the green beans because I didn't like them, can't blame them. The oatmeal I tried before I started feeding FF, so it may have been the sticky wetness that made them avoid it. I did give them some leftover crockpot steel cut oats with apples and dried cranberries that they didn't object to in the slightest.