Making Wii Sports Tennis burn more energy
Solution 1:
The only way you'll burn reasonable calories in any of these games (Kinect games included) is to hook everything to a stationary bike, and pedal to keep your TV either powered on, or descrambled. You'll never hit your target heart rate playing Wii Tennis or Mario Kart.
EDIT -- perhaps another approach is to tie your Wiimote to something bowflex-like, so that you have to exert more energy to swing. :) Not sure how you could actually implement something like that, though. I imagine it would be okay if you were only hitting forehands, but switching to the backhand side would probably be challenging.
Solution 2:
Just stand up.... here is an article about the benefits standing up at your desk at work... while you are asking about games I imagine the effects would be the same.
Standing up at work