You don't need the bag around the waist, the balance board cover, or even the yoga mat that some places sell. All you need is the Wii Fit; it includes the game itself & the balance board. (You need the Wii too, of course, and a Wii remote to make menu selections.)
If you have carpeted floors, that's all you need. The balance board cover is not necessary unless you feel that the balance board is a bit slippery. (I don't recommend wearing socks when you use it; go barefoot or else wear sneakers.)
If you have hardwood or any uncarpeted floor, then for YOUR comfort (not your balance board's), you could get the yoga mat - although any mat will do. The only difference is the ones marketed for the Wii Fit is large enough to hold both you and the balance board on it... so you can step in front of, beside, or behind it with no problems.
As for the waist bag, you can just as easily put your wii remote down when not using it, or else shove it in a pocket (or your pants' elastic waist) if you need it on you for the running portion.
you only need the Wii Fit