What's a good way to exercise a Boxer?

Agh. So I have a German Shepherd, Boxer mix, and he is just a bundle of energy. I swear you could play with him all day and he'd still want to play. I'd love to go hiking or running with him, but I live in a city, and there's no good places here to take him to that aren't very far - (especially with the weather getting warmer, I don't want to go off too far).
It's usually rather difficult to run with him; whenever I run a lot with him, he starts attacking my feet. He even almost tripped me earlier today when I was sprinting. (I ran into a trashcan though. No falling for me.)
I was thinking about bringing him across the street a ways to my highschool, and play fetch with him or something on our stadium football field. (We have dogparks here, but I don't want to have to worry about him getting in a fight with another dog, or getting any sicknesses.) The field is fenced in well, so there's no way he can get out.

Anyways, back to my main point; for all those Boxer/Boxer mix owners out there, are there any effective ways you have to tire them out? I'm running out of ideas. :p