A hammam right in Wadi Musa, roughly 200 metres from Petra's main entrance. The format is the traditional sequence: warm room, steam room, scrub and foam massage with a loofah and kessa mitt. After a day of walking the siq and climbing to the High Place of Sacrifice, it's a reasonable way to recover.
The building also has a restaurant on the ground floor. Convenient location if you're staying nearby and want a proper soak after the ruins.