This 5-In-1 Fitness Bundle is one of the best Wii Fit Accessories to get as it comes with rechargeable battery pack, USB charging cable, jeli sleeve, fitness mat, travel bag, and 1 pair of textured foot socks. What else could you ask for?
Here is amazon has to say about it:

Product Description
The 5 in 1 Fitness Bundle for the Wii Fit is the perfect accessory kit to accommodate any Wii Fit workout session. You will find everything you need to maximize your Wii Fit experience. The Textured Foot Socks provide comfort and extra traction. They are made from high quality cotton and the bottom is made from a strong rubber material. The Travel Bag is ideal for taking the Wii Fit Balance Board on the go. The Travel Bag is made from a strong nylon material, which makes it extremely durable and tough. The Jeli Sleeve is essential for keeping the Wii Fit Balance Board safe and free from dings and scratches. The Jeli Sleeve fits snugly over the Wii Fit Balance Board. It is made from a rugged rubber material, which not only provides safekeeping, but excellent traction and comfort. The Fitness Mat is perfect for added support and comfort. The Fitness Mat minimizes impact to your flooring and body. Add extra stability when using the Wii Fit Balance Board or use the Fitness Mat by itself when doing yoga and stretching exercises. The Fitness Mat is made from polyvinyl chloride (PVC). This material is extremely durable and will provide years of extended use. The Rechargeable Battery Pack & USB Charging Cable provides the necessary power to keep the Wii Fit Balance Board and maximum strength. Simply charge the Battery Pack whenever power is diminished. A fully charged Battery Pack will provide hours of use at maximum power.

Three reason why you should get a Wii Fit Balance Board Mat:

1. These Wii Fit mats allow you to have a soft cushion behind the actual Wii Fit, so that when you are stepping on and off of the mats, your feet won’t hurt.
2. Someithing else that these Wii Fit mats do for you is provide an anti-slip surfaces so that you don’t fall when rocking out the fun Wii Fit games.
3. Lastly, you can also use the Wii Fit mats to keep your Wii Fit all nice and clean.Bling-Bling!  With the possibility of so many different peoples feet on your Wii Fit Balance Board this will keep other people’s funk (and diseases… just kidding, I hope) off of your balance board.

Here are some great Wii Fit Balance Board Mats:

Wii Fit Balance Board Mats

Wii Fit Balance Board Mat	Wii Fit Yoga Mat
Wii Fit Yoga Mat and Ankle/Wrist Weights Combo	Wii Exercise Mat Designed for Wii Fit

Wii Fit Games (and games that use the Balance Board)

Skate It

Shaun White Snowboarding Road Trip (This is a SWEET Wii Fit Game)

We Ski

Skate City Heroes

Jillian Michaels Fitness Ultimatum 2009

Active Life Outdoor Challenge (This isn’t made for Wii Fit, but most Wii Fit users love this game)

Rayman Raving Rabbids TV Party

All Star Cheer Squad

Wii Music (Get some Wii Fit Music in your life! )

Wii Fit Accessory Bundles

Wii 5-In-1 Fitness Bundle (Includes: rechargeable battery pack, USB charging cable, jeli sleeve, fitness mat, travel bag, and 1 pair of textured foot socks.)

Wii Fit Workout Kit (Includes: nonskid workout mat, rechargeable battery pack, charging cables, two soft grip 2-lb dumbbells, workout towel, 2 wristbands, water bottle.)

Wii Fit Rechargeable Batteries

Wii Fit Rechargeable Battery Pack for Balance Board

Wii Rechargeable Battery Pack & Cable For Wii Fit Balance Board

Wii Rechargeable Battery Pak Designed for Wii Fit

Large Capacity Rechargable Battery Pack for Nintendo Wii Fit Balance Board

Wii Fit Energy Pak

Wii Fit Balance Board Sleeves and Covers

Wii Fit Balance Board Clear Silicone Sleeve (Comes in other colors, such as…)

Wii Fit Balance Board Glow in the Dark Silicone Sleeve

Wii Fit Balance Board Blue Silicone Sleeve

Premium Pink Clear Durable Flexible Soft Silicone Skin Cover Sleeve for Nintendo Wii Fit Balance Board

Wii Fit Balance Board Mats

Wii Fit Yoga Mat

Wii Exercise Mat Designed for Wii Fit

Wii Fit Balance Board Mat

Wii Fit Starter Kit (Yoga Mat + Silicon Sleeve For Balance Board)

Wii Fit Sport Cushion Mat - Very Soft

Wii Fit Balance Board Exercise Mat - Yoga Mat

Wii Fit Carry Cases

Wii Fit Travel Bag

Official Wii Fit Balance Board Sleeve

Travel/Storage Case Designed for Wii Fit