Flock Block, suet baskets stuffed with veggies, hang a Farmers Helper "HotCake", pumpkin cut open...I also got a "Hentastic Treat stick" from Jeffers as a freebie that I am going to try. If you plan ahead, plant some forage seed in some planters.....I had 2 and just gave them to the birds during the snowy days. Didn't last long but they seemed excited about the bit of greens. Also, being a crazy person, I bagged up clean, dry leaves from the fall....I dump a pile at one end of the run during REALLY boring days for them to kick around in. If you have room, put some big, rotting logs in the run too...they will spend a lot of time picking at them.
My indoor chicken LOVES her treat ball....keeps her quite occupied as long as I fill it daily.