Mine are in confinement full time, so I've also been giving a lot of thought on how to provide greenery and plants to munch on.
One idea that I've yet to try is putting lemon balm or catmint (both HUGE favorites) in an outdoor planter, then putting a garden cloche over it so they can only nibble what grows outside the mesh. I'm sure they'd have it trimmed down in 10 seconds, but it would give them something to look forward to every day.
I do a similar concept outside their pen; we built a garden box right up against the hardware cloth, and the plants will gradually grow through the sides for an occasional treat. We have lavender, rosemary, and catmint. All do well in full sun, and don't really need watering (assuming you get some rain).
I've also started growing wheat grass sprouts indoors. It takes about a week to grow to 3-4 inches, and it's quite easy. The Chicken Chick has a nice article on her website.