Try not to give them too much as it leads them to eating that then there feed which is actually healthier for them,but sometimes when there bored I hang lettuce on the roof or feed them live meal worms.
Mine turn their nose up at such things in most cases. They do, however, get the cobs from sweet corn on the rare occasion that we have that for dinner. They go nuts over those. They also enjoy it if I tear up a handful or two of green grass for them or give them a handful of hay to dig around in.
Mine have been enjoying baby spinach leaves, cut up (tiny pieces) apple. I have noticed their shells are a pretty green hue under the brown splotches. I will have to look up the nutritional value of such a dark green leaf veggie. My apples are in a refridgerator from this last fall from our orchard. Too rubbery for us humans, but the quail and chickens are enjoying them as a supplement or treat. Good way to use everything we grow.
Heck I would plant a plot of dark greens just for the quail and the chickens (separate of course) just for the value it gives for the rest of the year.
I also feed alfalfa fines (the tiny broken but dark green leaf part of the alfalfa bale). Walmart and some of the bigger pet stores sell bags (mini bale) of alfalfa. I reach in and grab the green fines and sprinkle in with their game bird starter or layer.
Mine love apples. Pretty much any kind of apple in fact. They also are big fans of sweet corn as well. They can pick an ear clean in no time. I also noticed that if you pull the center out of a cornstalk before it develops ears they love that tooo. Finally I think their all time favorite is chickweed. Its like quial-nip!
Like it's been said on here time and time again, obviously, "treats" can be a bad thing to a certain degree, and this pertains not only what that "treat" is, but the frequency as well. But as for me and my Bobs, their "treat" time consists of maybe a partial head of lettuce......or a partial bag of carrots.........or some old musshy maters.......about............ohhh.......once every 4 - 6 weeks (grown birds only). Other than that, they're gettin' Nutrena GB Breeder - Finisher.