Indoor rabbit run
IKEA Kallax expedit DIY cage for my guinea pigs and hedgehog. It is 17.1 sqft total for three levels; which is larger than the suggested minimum size for 4 cavies. Mine can run full tilt and love to do laps up and down the levels; this dispels the idea that each level is too "small" because they can't run. It's still bigger than a 2x6 C&C cage and all the same natural behaviors are exhibited.
runaround.co.uk tunnels for joining runs to housing - bunnies love them plus you can move the runs around on the grass. This run, in itself, is not very large but it connects to a large shed via a long tunnel giving plenty of other space for the bunnies to scamper around.