Fruit tree sticks
Little bags of fresh hay, with just a handful in at a time so that it can be changed every few hours so it's always super-fresh, which encourages constant nomming and keeps them entertained too
Hanging those wooden chew toys (about 2.99 for 3) off old shoelaces from the roof
Things to jump on (Ludo is indoors so I took the cables and anything dangerous off my old printer and he uses that)
Ludo did used to like a cardboard box but we had to confiscate it as he ate too much of it and he's stasis-prone, but a lot of bunnies get on alright with cardboard