Originally Posted by christep
Most of the 5* hotels will sell you a day pass to their gym, particularly if you only use it during off-peak hours. So it depends exactly where you are staying in Kowloon - just go to the most convenient 5* hotel and ask.
Fitness First Plus at the top floor of the Sheraton has a good gym, pool, and spa facilities. What you can do is call them first, say that you are interested in joining as a member and that you want to get a week-long pass to try out the facilities and see if you like the place.
They will normally give you access 1-3 days for free to evaluate the facilities (you have to sit through a sales pitch and give your business card) and get a discounted rate for the remainder of the time.
I've done it before.