Walmart has a big bag of "greens trio" for less than $3, put a couple layers of paper towels in the bottom of a plastic container, fill it with the greens, it stays fresh, and has enough to feed a large adult daily for a couple weeks. I'll add small pieces of other fruits or veggies we eat for a little variety, you can finely chop the greens, but mine eats the coarsely chopped leaves as they come out of the bag. He'll munch on the greens throughout the day in his cage. I take him out and feed dusted superworms by hand as I handle him, seems to help increase trust and helps tame him.
My two love eating a combination of mustard greens, turnip greens, and collard greens. Sometimes I throw in some kale, not a lot but just a little. I just shred it up in a small strainer, wet it down and toss it. They eat it like no tomorrow!