I've used sticks and leaves in my garter cages. They loved them, I'd bake them at 200-250 to get rid of bugs. Leaves start to smoke after a while, even at that low temp, so you have to keep a close eye on them. Garters loved to "play" in the leaves and loved climbing sticks.

You can also bleach and or boil the wood. For my dart frogs, I did all three because I feared the moist environment would leach things out of the wood. Bleach soak for a few hours, boil for a half hour, let dry overnight, then bake off anything left. The baking was probably overkill, but it wasn't labor or cost intensive, so I figured what the heck...