I find they like any sort of seed - cucumber, zucchini, squashes - seems they go for those first then eventually get around to the flesh. Can't remember where I read it, but I seem to recall cutting a squash in half and filling the inside with yogurt. I haven't yet tried that, just now remembered it. Maybe whirl some herbs into the yogurt.
During the winter months, I often hang a cabbage off their coop so they get some greens (and a bit of exercise if it's high enough that they have to hop for it).
