Bunnings sells $1.00 sea grass mats. They do come from china, so they have to be washed in hot water and thoroughly rinsed and dried in the hot sun before use as a precaution (if you know your bunny uses them it's good to get a bunch at a time when you know there's going to be a really hot day so they dry well). But they're great fun for chewing and digging at, and if you curl them around and tie the two ends together with a cable tie you can use them as a tunnel. While you're in bunnings, don't forget to get a cardboard box for bunny to play with too
Otherwise, I've bought cheap stacking cups and plastic baby keys from big w and similar places and bandit has fun tossing those around. Otherwise toilet paper tubes etc are always a good standby as well the white pages (cover ripped off).