BALL!
We used to joke that as far as Moree was concerned, we existed for three reasons, fetch, food, and pets, in that order. He loved to play fetch, and would do so to the exclusion of all else (unless we were at the dog park). When we first got him, we would occasionally have to put the ball in his bowl of water because he would not, would not, take a break to drink, despite the fact that his tongue was hanging down to his knees.
As he got older, we were able to teach him the phrase “we’re done”, because we were always done before he was. He would then take the ball and lay down for a few minutes before coming back and trying again.
Also, if we tried to put the ball away, and he saw where we put it, he would remember. If, a few days alter, we said, “Where’s your ball?” he would go exactly to where we left it and look expectantly. If it happened to be in one of the drawers under our bed, he would open the drawer and pull the ball out.