Grass clippings are only ok when they're from a manual mower, the turning blade/gas free type.
Anything that went under a blade comes away with chemicals, oils, exhaust debri... yucky stuff for animals. Goats, horses... they can't have it. I'd imagine it's the same for chickens.
Anything cut with a string trimmer should be alright.
BUT, chickens graze off of rooted plants, because it allows them to grab off what they want, to avoid large pieces. Clippings tossed in would need to be cut shorter (to 1 inch lengths tops) or avoided.
But definately keep at the veggie store for melons and such... green peppers mine seem to like the hearts of, they also munched down on broccoli heads.
Maybe add in a stop at a bakery to try and score some whole grain day-old breads. Avoid boring bleached flower bread... wheat or whole grain crumbled up into small bits is a good treat.