The main sight of town is without a doubt the mosque of Djenne, but do wander about town a bit to explore the other interesting things.
The Mosque the grand mosquee de Djenne is basically a big mud structure, the biggest in the world in fact, but it should not be missed under any circumstances. It is a curious attraction, but absolutely worth a detour.
The mosque overlooks the market square. The strcuture itself is not very old, and only dates back to the beginning of the century, but it was made on the same spot earlier mosques occupied.
According to some reports tourists are not always allowed in. Be sure to dress appropriately and behave respectful. This is a place of worship.
Market Monday is market day in Djenne. People from the area around come to town to buy and sell goods. It is great to walk around, to look at the goods (the cloth, especially), and to haggle. People are friendly and easy going generally speaking.
Madrassa There is a big number of madrassa's or Coranic schools in Djenne. They are not always very easy to find but a local guide can show them to you and explain about the teaching methods that are still in use.