We hinged 4 pieces of plywood and fixed them to the wall on wooden batons either side of a corner of the room so they fold back like a concertina leaving the corner open while the buns are out and about, 3 doors have squares cut out and mash fixed in, one is cut in half like a barn door so I can put food in without being tripped up when they start running circles!
We've been able to build little wooden houses for them, put grass boxes in on lino and fix batons onto the wall to hang string from with boxes and loo rolls attached to them and have the base of a ferplast 140 as their little try in the corner. It is a fairly permanant instalation as it'll need a bit of pollyfiller on the walls if we ever take it out but it's the perfect base for them and I don't feel like it takes up too much of the room.