Ash Shawbak Castle is located in the Ash Shawbak district and you have the option of visiting it on your way to Petra.
Shawbak is a crusader castle built to control the caravan route from Arabia to Syria. I was able to wonder around quite happily for around an hour completely alone in the entire castle. Pretty awesome stuff when you think about the lives lived there hundreds of years ago.
Entry is free and is well worth a look!