You can put a swing in it, look up chicken swing on Amazon. Other things would be toys for parrots, a little tykes toddler slide, or the toddler cube that has the slide with it. A small soccer or basket ball, a xylphone attached to the run or coop, other musical toys like drums, cow bells, a toy piano, anything that makes noise when pecked. Things that move when they peck it should also work. Basically, as long as they can't easily destroy it, anything that a child up to about 2 yrs old would play with that you think they would enjoy. You can even use old wind chimes, just fix it so they are lined up on a string or loosely nailed on a board so they can make noise when pecked.
I have a lot of ideas, it's just not all that easy to describe them all. Basically, look at things like a kid who wants to explore and touch everything. What would be fun, and then think of all the things that could be choking hazards, could it be dismantled/destroyed by a beak, and if so, will it cause damage to the bird, or can it be swallowed.