Screw Wii Sports and the Balance Board. If you really want to sweat your weight off you only need a NES, a Mike Tyson's Punch-Out! cartridge and a properly wired boxing dummy:

I wanted something I could actually play as if I was punching the person I was playing. This led me to modifying my Slam Man Boxing Dummy to add arcade buttons I could punch.

The hack itself is quite simple, but ingenious and definitively effective for both the gameplay and doing some actual exercise (boxing is one of the most taxing sports I've ever practiced). [Instructables via Make]