In my last bunny post, I mentioned Noe’s “bunny palace:” a bike box cut in half and stacked with another box to make a 3-level structure. You can see only two levels in this photo, but here it is. There’s a smaller box on top, but it’s cut off in this photo.
Notice that the levels are perfect bunny-height: tall enough to hop around in, but short enough for a tunnel-like feel.
Here she is on level 2, munching on hay:
Having several entrances into each level is a must.
You can also buy specially-made cardboard bunny houses, but my philosophy is, as long as you have a pet that loves to play with old boxes, you should make the most of one inexpensive option.