How to exhaust one gerbil :
Give him an entire toilet paper roll to play with / destroy.
Poor little guy, fell asleep beside it. He really did his best to destroy the whole thing, but only made it about halfway through burrowing into it, then managed a spray of white fluff as he attacked it tooth and claw. He came out to spread the new fluff around and ended up just curling up where he was, without even burrowing.
I'm sure he'll be a right trooper and continue on with his task after his sleep. :D