Merkato is one of the largest markets that even the most well-traveled have ever seen, and is rumored to be the largest market in all of Africa.  The best places to go include perfume stands, the spice market, and the "recycling area" (where all finished products for sale are created from discarded tires, bottles, paper, and other undiserables).  There are an estimated 13,000 vendors working in the market on any given day.  Retreat into a restaurant for local food if the scents, sounds, and crowds become too much.  The Al-Anwar mosque is nearby.