Take a break every 20-40 minutes, and do:
roll hips a few times clockwise, and a few counterclockwise
roll ankles a few times clockwise, and a few counterclockwise
roll shoulders a few time clockwise, and a few counterclockwise
10 frog pushups
10 calf raises
roll arms 10 times one way, 10 times the other
look off more than 100 yards to relieve eye stress (at least 30 seconds)
repeat if you need to.
Throw in some burpees and handstand pushups against a wall, if you want.