We looked at the Spa and Fitness Center yesterday. The Spa is amazing, but small. The Fitness Center is a separate door over to the right, and you get in with your Magic Band. I am pretty sure that both Grand Floridian magic bands and Poly magic bands would work. The fitness center has a great selection of Nautilus-type machines (but some other brand) and is quite large. When we peeked in, it was empty, but I bet it gets busier during non-park hours.
There is an outdoor basketball court available as part of the Fitness Center.
The Fitness Center has no restrooms or locker rooms - they are not allowed to use the restrooms in the Spa. But the Beach Pool is not too far away, and you can change and use restrooms there.
I wish I had taken a picture.