I think you can get a rubberised ball - heavier than a tennis ball so you don't get a 6 off every ball, but not as solid as a leather one so you don't break bones

Indoor might also be OK with a tennis ball, as it will ping off the walls and not go for miles

If we do have a harder ball then gloves and pads are probably going to be needed, which means we'll probably have to try and get some time at a "proper" cricket club - I'd rather just do it as a knock about where it's more informal though?