Buying gifts is always difficult I think.
You want, of course, make sure that the person likes it and uses it. You don't want to buy something that is put in the wardrobe and forget about it.

So what do you buy someone that is interested in fitness?
Well you have an endless amount of options actually.

Some suggestions: A Complete pregnancy fitness kit.
Or, something for the kids:Fitness Fun for kids.

A good fitness book, or why not two, three...
Maybe the person you are buying for likes to play tennis? then someTennis accessories will be the right thing to buy!

You are sure to find something nice to give away at Motivators.com. Check out their promotions!
If you are thinking about buying fitness clothes or something more personal why not check out the things over at CafePress. They have loads of different nice designs to choose from.

You can also give away a Magazine Subscription, a workout DVD, some good music that makes you want to move, gift certificate, a gym membership, a personal training session.
Just use your imagination and you will surely find something nice!