Looks like a scale. There are tons of games for stretching, etc.
Wii Fit(TM): This experience builds on the active-play phenomenon started by Wii Sports(TM). It comes packed with the pressure-sensitive Wii Balance Board (name not final) that measures users' actions in fun aerobic, toning and balance activities. Wii Fit also promotes communication about health among family members, as everyone can review one another's progress on a new channel on the Wii Menu. Coming in the first half of 2008.
Jason Chen: First demoed is one leg stretch, where you stand on one leg and stretch. Maintain balance to strengthen your legs. 10:04
Jason Chen: "We thought the Wii was a device to be placed in the living room", and should be accessible to everyone in the home. And the one application they need is health. 10:03
Jason Chen: Miyamoto says: "The one game I wanted to introduce to you at E3 was WiiFit (not Mario Galaxy)." 10:02
Jason Chen: Videos' over, and three fit-looking people are on stage, along with Shiggy Miyamoto. 10:01
Jason Chen: Other games, pushups, Yoga, soccer games, dancing games, all look like they use the pad/mat/whatever this thing is called. Name will show up shortly. 10:00
Jason Chen: The last game they're revealing, Wii Fit, features a new accessory: a scale-like device you stand on. There's some workout demos on screen now.