My playhouses are in my garage so don't need to be fox proofed but I know a lot of people fit second split mesh doors inside so they can leave the shed door open in hot weather but the tough mesh keeps out predators and the top part can be opened so you step over the bottom part to get in without bunny getting out.
Mine have tubes, boxes, toilet rolls and bunny toys inside to keep them amused as well as an attached running area.
Caz