That ball pit is a really good idea, might have to invest in one myself. At the moment I use one of the metal outdoor runs from P@H inside, but it is a pain, as has no floor, so need lots of layers of paper, waterproof sheets and fleece to porotect my floor. As there is a built in floor in that ballpit, fleece and towels would be fine.
Do you think there would be any way to rig up their water bottle on the side somehow, mine spend most of the day inside in their run in bad weather (most of the time it seems with this weather!), and they always spill the water if it's in a bowl on the floor!