This is a good theme for a surprise party. Plan it for right after an exercise class or following a game night in a sports league. The guests will be the members of your class or team. This has to be done on the sly, so have ribbon, balloons, and streamers ready in people's lockers.
Some fitness centers have meeting rooms or cafes, which can be dressed up nicely and in a hurry.
The key is surprise, so the decorations can be sparse. Gifts Workout clothes, sports equipment, gift certificates to classes, tickets to a game, a juicer, health food cookbooks. Party Favors Sporty water bottles or fanny packs.