You don't specify the exact hours you have, so it is hard to answer.
I would say that the the Roman ruins in Jerash, 30 miles north of Amman, are a lot more interesting than the Citadel in Amman, should you not have time to do both.
The baptism site can be time-consuming -- you can't go on your own, instead they have guided tours every few hours -- so it might not work but is interesting if you have the time.
You could do the dead sea -- the public access beach is good. Dead Sea and Jerash would probably be tight.