Debre Sina is a highland monastery east of Keren, important in Eritrean Orthodox tradition and associated with some of the country's earliest Christian history. The journey is part of the experience: the road climbs away from Keren's bowl of dry hills into rougher upland country.
This is a religious site first and a sight second. Go with a local contact who can explain access, dress conservatively, and expect simple conditions. If you are in Keren for more than a market stop, Debre Sina gives the surrounding landscape and Eritrea's Orthodox heritage much more depth.