Baby rings, hammocks, pigloos, sputniks, hidey houses, wood chews, bird toys, cat balls, PVC pipes, dollar store baby toys/baskets, dog ropes, ladders, lava ledges. Those are some of the things I have used.
Baby toys are great! I bought some at GoodWill for like 15$ ( I bought 2 huge plastic house hide-aways, a basket, and a huge 5$ bag of tons of baby links and other cute baby toys)
It was super cheap and super worth it. I can't even fit all the toys in so i change it around every time i clean the cage for extra fun!