For a day by the water with a difference, check out the bubbling pools of Ain Al Kasfa. This spot is renowned for its mineral-rich thermal waters.
You're spoiled for sightseeing choice when in Rustaq. Ain Al Kasfa is a good place to begin here, but there are lots of other worthy spots to visit. Abu Bakr Mosque, and Rustaq Fort are close to the naturally heated water source and will round out your day nicely.