PS. The Wii Fit is a joke, about as useless as a Bowflex machine. There are much better ways to spend your money than that.
Edsta, I generally have agreed with most of what you write/suggest but you are way off base here. While the Wii Fit may not be the best for an advanced fitness person, but it is great in this instance. You are taking a teenager that was not very active and starting her on the road to fitness. In this case anything that gets her moving and active on a regular basis is a good thing. The fact that it is fun will keep her at it and allow her to make activity a habit. That is the most important part at this point. She has to develop activity, fitness and health as a habit in her daily life. There will be time later on to participate in more beneficial activity later once she gets comfortable with the new activity level.
Your suggestions of Krav Maga or Thai kickboxing may be a little over the top in this particular instance. It is very difficult and a little overwhelming to take someone in this girls situation and immerse them into something this demanding. You don't want to overwhelm her with intensity or difficulty when she first starts. This could discourage her. Once she has increased he activity level I think it would be a great idea. However, given her situation I think you might suggest something more incremental.