I've fed celery leaves, bean sprouts (technically a protein, but who doesn't like sprouting beans), chia/flax seed sprouts (not a veggie but fun to grow), carrot shavings unless the population is large enough to finish a carrot in 2 - 3 days, zucchini, squash, pumpkin, swiss chard, spinach, kale, romaine lettuce, radish, etc. Any extra fresh veggies I toss in. Most everything has been more popular than potato (potato traps are so deceiving lol). I haven't tried mushrooms yet, but I've observed that bell peppers are quite popular. (:
But they especially enjoy meats, pinks/pinkies, snake shed, fish scales, cicada exo, banana peels (I just bury it in the substrate once it blackened completely), and the biggest hit of all dead roaches with many of my species.