Ajlun is compact enough that you will combine it with another destination rather than treat it as a base. The natural pairing is Jerash, 20 km to the east — one of the best-preserved Roman cities in the world, and easily done in the same day.
Anjara, a few kilometres to the north-west, has a Greek Orthodox church built on a site where tradition says Mary rested during a journey through the region. It is a short detour and appealing if you have a car.
Irbid, Jordan's second city, lies 30 km to the north and has the Museum of Archaeology and Anthropology at Yarmouk University — a worthwhile stop if you want to put the region's ancient sites in broader context.