Dave,
I generally just take the night off since it seems like I am shooting something 5-6 days a week anyway.
If I decide to go cover something without a real reason, I usually pick something close and go try and shoot in a way that I normally don't. Ex. Panning, odd remote camera setups and locations, video with the 5D Mk II, experiment with new strobe placements, etc. I love trying new things but love it more when there is no pressure so if everything turns out like crap it is no loss.
The other thing I will do sometimes is go cover a team I cover on a regular basis and shoot anything going on besides the actual game. Ex. Go shoot from the stands and talk to the parents, the people working the concession stands and taking tickets, the kids playing around the field not paying attention to the game. I followed the AD around one night and shot what he did for a night and made a slideshow of it. That sort of stuff. It is a good chance to get to know the people and have a more recreational evening.
Hope that helps.