In my opinion, I’d wear either 1) a conservative dark-colored suit, hose, and low heels OR 2) nice fitness/gym attire, i.e. tennis shoes, long pants, a shirt (no tanks or anything revealing), and a zip-front jacket.
Of course, my first suggestion would be to ask the person you spoke to when you scheduled the interview what he/she recommends you wear. I’ve done that before and that’s totally acceptable; in fact, they appreciate you taking the initiative to ask. As Karin pointed out, some industries are more conservative in their dress codes. Staff at corporate headquarters are often expected to dress a little more formally than those at branch offices, but that’s not always the case. You can also research the dress code for a particular company online if you don’t feel comfortable asking HR directly.
I’d choose one end of the spectrum instead of riding the fence. If you’re in business casual and another candidate with similar skills and experience wears a suit to the interview, you may not get the job. I don’t think you can over-dress for an interview. A suit says you made an investment and took the time to impress, and your new boss obviously realizes you won’t be wearing a suit every day if you’ll be teaching fitness classes. However, I think you could have equal success wearing a more polished version of your everyday gym attire. In my mind, there’s a disconnect between business casual and fitness. Even if the staff in the office wears business casual and might look dressier than you when you interview, chances are that if you get the job, you’ll never wear business casual—you’ll always wear clothing appropriate to your job. If you’re more comfortable and confident in fitness attire than in a suit since that’s what you wear every day (as I am), you may do your best interview in your normal work clothes as opposed to a suit. You’re better off looking self-assured than self-conscious no matter what you wear, so keep that in mind.
Either way, I agree with Karin’s suggestion that you wear minimal jewelry, very little or no perfume.
Best of luck,