Very random but we actually bought an old parking meter from the city. When DS has completed whatever we need him to do, in the allotted amount of time, he earns a quarter. He can either keep the quarter for his piggie bank or use it to "buy" video game time in the parking meter. We have a small bucket near the meter where he can store his quarters and save them up. Then I'm not the bad guy declaring it's time to turn off the game- the parking meter says "Expired".
On that same note, if I have to do something for him that he should be easily able to do for himself (example: leaving dirty laundry on the floor) then he has to pay me. I charge a nickel every time it's something small and a quarter if it's something big (like picking up the lego room after I already asked once).
His currency is... currency!
