Dog 'Boomer' type play ball (Highworth SN6)

Similar to the Boomer brand, it's 24cm in diameter, hard plastic and unless your dog can use a sledge hammer(!) - indistructable. Through the very secure screw-in stopper can be part filled with water to create a wobble effect when pushed around. With years of life left in it, it's amused a labrador and a doberman plus their smaller visiting terrier type friends in the past.