hamster ball tied to the side somewhere. you can often find them in flea markets, yard sales or used discount shops.
another way to make cheap and quick double purpose levels is dollar store dish clothes and towels (you could also use old clothes or bedding-ask around for it and you will be up to your ears in it in no time) and safety pins. you can fold them into pocket hammocks, corner hammocks or stretch tight for easy to clean and bouncy levels. cheap tunnels can be found in PVC pipe. if there is construction going on somewhere handy, hang out there for scraps. make sure you wash the heck out of it but free tube! bait bags are also great fun for them. take some cloth, string and treats. place treats in the middle of the cloth and bunch the cloth around the treats to make a bag. tir the string to the bag to close then tie the other end from the top of the cage. they will have fun trying to get the treats out and something tasty once they do.
also, keep your eye glued to the classifieds for cheap cages or come-and-gets. another great way to find free cages is taking a walk around (or a drive) come garbage day. it is amazing what people will just throw out. there is a post in the habitat section of this forum about "beni's retirement home", i saw that exact same cage, no damage all pieces intact in the garbage a few weeks ago near where i live. i didn't get it because its kinda heavy and i didn't need it, but it was just lying there for someone to take it. go out early though, there are a lot of other divers out there as well. i got my futon frame this way, went to a pawn shop and got the cushion for $10. in stores it would be nearly $100 and there is nothing wrong with it.
when money is tight you need to learn how to scrounge. but once you learn it, it is an valuable tool. not everything that is thrown away is trash.