Probably been done before, but coconut fibre (stuff used for nesting and whatnot, can be bought at numerous pet stores) stuffed into a little plastic cat ball thing (with random fibres sticking out) seems to be quite the favourite. You can tie a piece of thread to it to hang it or pull around. Hopefully some of your birds will enjoy it. By the way, the coconut fibre is sterilized and dust free, safe for your birds. Let me know if any of your birds enjoy it as much as mine. May even want to stuff a bit of millet in there too. Treat and a toy.