I have started taking a friend's 5 year old boxer to the beach for
walks. She's a fairly reserved dog, plays a little with other dogs and
stops to sniff people. But really the only thing she wants to do is
play fetch with a ball.

Once we start playing, she'll hover around me waiting for another
throw even as long as 10 minutes or so after I've put it away. I'd
rather she went off and did something else (play chase with other
dogs) so I could watch for a while.

The past week I've started doing 10 minutes ball/10 minutes no ball
and this "kind of" works. She's less agitated when I stop playing.