Do change the water frequently though and avoid bowls where there are baby guinea pigs as they can drown. Hay racks help prevent waste and soiling but watch out that babies and rabbit legs don't get caught up in them. An extra hay rack, if there's room, is a great place to put vegetables, grass or wild plants as it makes the animal work harder for its food. The bigger the housing, the more room there will be for toys. Apple and pear tree prunings, willow wands or hazel twigs are good for gnawing on.
Guinea pigs like to hide in cardboard boxes or tubes.
Do make sure that if you use tubes, cardboard or plastic, that guinea pigs can't get stuck in them. Given room to do this safely is their most basic requirement.