Just to address a few questions...
The Wii (around $260-300 depending on where you get it) comes with the Wii Sports disc which includes bowling, golf, tennis, baseball, and boxing....and the ability to give you sore muscles where you didn't even know they existed. It came with one wireless controller and we purchased an extra one (which I would advise for family togetherness).
Wii Fit was an add-on which cost us $89 (happy Mother's Day to me). It comes with yoga, strength, balance, and aerobic exercises and we haven't purchased anything additional to go with it. It weighs you, tells you your bmi (who knows how accurate it actually is) and does a body test any time you ask it to, in order to see how your balance is, etc. It keeps track of how much you've done, how many days.....tells you when you seem like you're getting too tired to do very well, etc. It will even tell you if it thinks you're obese. Yikes. Thankfully I haven't heard that yet. A year ago, I would have.
I still haven't exhausted all the games that are in Wii Fit. It rewards you by giving you new games and exercises for every large chunk of time you spend on it. So far I've got snowboarding, slalom, ski jumping, hula hooping, boxing, aerobics, about 10 different yoga moves, about the same amount of strength exercises, jogging, and a few other oddball games.
Again, if you're an athlete's athlete....you might roll your eyes at this but I think it's pretty cool.