Apologies if this is already known, but i've been hanging out on this one stretch of land off of the food court (I think) where the hardware store and gym are.. racked up nearly a thousand zombie kills here in like 20-25 minutes but that's besides the point..
If you go in the gym and hit the punching bags with something like the weights (or just ride a bike through them), you get 2,000 PP for each destroyed bag.. so 2,000 x 4 bags = 8,000 PP, then you get a 10,000 bonus for destroying all four, so 18,000 PP for about five seconds worth of work. Just found it out.