What toys do your rats enjoy most?

A question for all you rat lovers: which toys do your ratties seem to enjoy playing with most? I'm looking for some new toy ideas for my girls, in particular, toys they can play with outside the cage. They already have numerous hammocks, fleece tubes, sticks, paper towels, cardboard boxes, swings, and a wheel inside the cage. But I need some better toys for them outside the cage. They seem to get bored easily so I'd like to provide them with some more mentally stimulating toys. Does anyone have any suggestions? What toys do your rats have the most fun with?