One of the main destinations during the Uyuni tours is the spectacular Isla Incahuasi (Inkawasi or Inka Wasi), an island in a sea of salt covered with giant Trichocereus cacti. It is famous for the spectacular panoramic views of the salt flat surrounding it. Most tour groups have their lunch here and there is also a cafe-restaurant that serves excellent llama stakes!
You can take a 15-minute walk to the top of the island, with a trail that loops back, and it is definitely worth it. At sunrise Isla Incahuasi casts a long shadow over the salar and the island is filled with colors.
Note that during the wet season when the Uyuni Salt Flat is flooded, the Incahuasi island is often inaccessible.