My girls used to love cat jingle ball toys. Penelope used to pick one up in her mouth and zoom around the cage with it, I would hear it jingling at all hours, day and night.
They also liked the cat chase toys on a stick, I had one that was a little feather boa at the end of a plastic stick and would shake it around and make it tickle them and they would chase, pounce on and bite it.
Then there was one I bought from Wal-Mart for a couple of dollars that was a walnut in a shell surrounded by a ball of something like dried grass/twig/straw material. They loved shredding them and then chewing open the walnut inside.
I also made a tunnel obstacle course throughout their cage of 4 inch diameter pvc pipe sections that they would chase each other through and hide their favorite things inside of.
They were big fans of those lava ledges, I staggered them all over the cage for them to play on and they got well used.
One last thing was the corks from wine bottles. They loved to carry those around the cage as well.