(Don't ask me how to make these cause I have no idea, but you can google tutorials!)
Hammocks
Tunnels
Pouches/pockets
You could cut loads of strips of fleece and put them all into a cardboard box with added treats and it makes a digging box! Plus they'll probably use it as a bed once the treats are gone and nest in the fleece (or chew the box to shreds like one of my girls did...)
You could make a bridge from one side of the cage to the other? Like a rope style bridge? Get three large strips of fleece and plait them together to create a balancing bridge.