I see @Owen has given you some good suggestions.
I hang cabbage, bundle of kale/greens (currently from the grocery), apple, etc. I also sometimes use a suet basket and stuff it with greens as well.
As mentioned, leaves and pine needles raked from the yard are a favorite. I rake up extra an place in large leaf bags (mostly pine needles) and store them under the porch for use in winter. On a cold dreary day, nothing beats a bag of pine needles for them to dig through.
If you have some property or a field that hasn't been treated with chemicals, you can also look for dandelion, plantain leaves, chick weed, etc., to toss in the run. There is also some winter grass in my fields, so I cut some of that with grass shears in winter/spring, but I do chop it up in small pieces. I also have bamboo that is green all winter, so I cut the tops out of some of them periodically. I hang a bundle or 2 in the run. Mine nip/eat the leaves and seem to enjoy getting underneath the bamboo branches.