Most hotels have spinning and bodypump classes. I have been spinning at the intercon near west bay lagoon which normally has almost full classes (nobody likes half empty spinning class), they charge abour QR35 and the sheraton have a similar one too, those are the two most popular ones with the hotels although almost all of the hotels will have these classes.

I joined fitness first because of the hassle free point of not having to pay each time. their spinning and body pump classes are good and they have a bootcamp class as well