Djenne claims to be the oldest city in West-Africa. Although this claim is rather hard to check, the city has a good pedigree. Founded in the 13-th century, Djenne was a good rival of Timbuktu in its heydays.
The main sight, the mosque is a construction form the 20-th century however. It is one of the most amazing sights in West Africa. It is the largest mud structure in the world, which means it has to be repaired every time it rains. See djenne/sights for more info.
Try to visit Djenne on a Monday when the town fills with the various tribes coming to shop at the weekly market. Djenne is particularly lovely during the rainy season (October-December) when Djenne turns into an island surrounded by a moat of water.