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. Make it clear on the invitation that it's a surprise!
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.
Workout clothes, sports equipment, gift certificates to classes, tickets to a game, a juicer, health food cookbooks.
Games and Activities
The guests will have just finished a class or a game, so just unwind, relax, and have fun! This shower should be quick (about an hour), so that the guests can then hit the showers.
Sporty water bottles or fanny packs.