TV is the primary reward. TV is like cash rewards for boy. Roughly $0.01 = 1 minute. Once a while, we can give him x hours of TV time as a lavish reward. Here are the immediate (or daily) rewards ranked by lavishness/extravagance:

1. Mobile game? I would say never a good reward. Too addictive and unhealthy
2. TV
3. youtube
4. a sugary drink or snack? not so healthy, but better than mobile games.
5. air-con? should be more liberal
6. shopping trip or after-dinner stroll?
7. downstairs play time? Should be most liberal.
8. new mineral water -> fridge. A bit wasteful.