Here are some fun things you can do with Wii Sports.  If you find others, please post then in our forums :

•  In bowling, release the B button during your back-swing.  The bowling ball will go backwards and all of the Mii's in the crowd will scatter.
  
  
•  In boxing, after winning your match, you still have control of your Mii in the background.  Move around, you can make the Mii dance and taunt the other player.