My brooder sits out in the run directly on the ground, so yes there's leaves mixed in. These are my current chicks, it's hard to see but there's a few crunched up brown leaves in there, along with wood chips, aspen shavings, hemp.
Lots of leaves elsewhere in the run.
You probably can, depending on what types of leaves. Wouldn't hurt to check and see if anyone knows if any specific leaves are toxic to chickens - I don't know any off hand other than poison oak/ivy being a no-no.
I have a mix of magnolia, willow, chestnut, maple leaves in my run.
Many of us deliberately expose chicks to outside contaminants, whether by brooding outdoors (like me) or bringing in dirt, to help inoculate them against things like coccidiosis via exposure.
10 days old and having a blast:
They'd rather have space to explore and things that allow them to do what naturally occurs to them, which includes the jumping and digging.
I don't think any toy beats giving them time outside in a safe enclosure, or bring the outside in by bringing in a pan of dirt and weed/grass clumps for...