[Tenchu]

Actually, Noodle, you might find French Boxing, Savate, to be the best for fitness. If they're training to actually compete in the sport, then they'll be doing a lot of conditioning, which means good fitness. But I always thought Savate was a joke compaired to Muay Thai, so...

As for what I think of all those other styles:

Aikido - More likely to be a lot of technical stuff, and only basic acrobatic motions thrown in. It can be good for a hobby, but I doubt will give you much fitness or practical skill.

Tae Kwon Do - Kicking uses energy, and thus TKD is good for fitness. Also, given it's a sport style, there is likely to be more conditioning thrown in there which is good. It is moderately practical style for real fighting, but you may need to work on your hand skills more than what they'll offer.

Jiu Jutsu - A lot of practical training and wrestling. Wrestling is great for fitness. The throws and locks you'll find very useful for self defence. Again, it has a sport to it, so is probably okay for fitness.

Fencing - More of a hobby, really. I've never done it, so I don't know how they train. You need agility to be good at it, which means fitness, but I don't know what they do. Though, using a sword I think would be fun, and no doubt the moving around will be basic fitness.

I suggest you do try 2 or 3 styles.