People always think of physically tiring their dog out but forget about mental stimulation. Think about it: if you exercise you're tired, but feel a bit energized at the same time. Now, if you sit down to work on something complex that you have to concentrate on, like writing a paper or balancing the checkbook, you're exhausted at the end of the day.

Have him play games with her that will challenge her intellectually. Hide and Seek is a popular one where you hide her favorite toy and ask her to find it. Get creative with the hiding places and make them easy in the beginning to truly challenging once she's good at the game.