I remember one Christmas my mom was hoping for some diamond earrings as a gift from my dad. Boy was she disappointed to find an exercise bike under the tree with her name on it.
Needless to say, the bike didn't get used much. Jazzercise was more her thing.
Anyway, I still think giving the gift of fitness is a great idea, but just make sure your friend or loved one has expressed interest in the subject. Folks tend to drop hints on these things or at least ask for advice on what gym to join. If you have heard someone say how much they would love to try a private session of Pilates - get them a gift certificate. If your sister thinks having a personal trainer sounds cool - purchase a session with a trainer at her gym for her. If your best friend keeps saying she wants to go to yoga with you, buy her a package of classes at your favorite yoga studio.
Gift certificates are an easy gift to buy, especially for folks out of state. Plus they always fit.