Midyat, about 40 min by minibus to north is also known for its stonework architecture, with even more ornaments than Mardin's.
Hasankeyf further north on the banks of Tigris River is a fantastic village with its cave houses, citadel, and other monuments from Middle Age. You'd better hurry on visiting the place as the whole town might be submerged in as near future as 2013 due to a yet-to-have-a-final-decision dam project.