I highly recommend World Gym at Al Shabab Club. They are currently doing a deal for AED 1,000 membership for 3 months. Includes gym, pool, steam, jacuzzi etc. The pool is 25metres long - the biggest I have seen in Dubai. The gym has all the latest Life Fitness equipment. Its not a busy club so you don't have to wait for machines. You can easily spend 2 hours there and not get bored or disturbed.