My rabbits don't care for zucchini either which I think is just WEIRD afterall it tastes better than weeds to me! I think they have eaten pumpkin/kobocha squash but those are higher in sugars. They often will leave the stems of kale preferring the leafy bits.
Note of caution: I have read NOT to feed rabbits brassicas - or in very small quantities - ie broccoli, cauliflower, cabbage, kohlrabi... because their digestion can't handle something in it -makes gas I believe. Probably just as it does for humans. Its actually advised for humans to only eat cultured brassicas, (cultured means broken down a bit like sauerkraut) but if that isn't possible, at least cooked not raw. Maybe safer for the bunnies if cooked also. I have not tried feeding them cultured. I have given them bits of cabbage and broccoli but only a couple pieces at a time, at long intervals.