The canyon itself. There are waterfalls (Berlin and Lisbon), the Pinnacle, God's Window, Wonderview, Bourkes Luck Potholes with as fascinating little display of the wildlife and geology of the canyon) and several viewpoints, the best-known of which is The Three Rondawels/Three Sisters.
Many locals sell artwork here, for example the famous wooden giraffes. Prices are higher at more frequented sites and lower at spots off the main stream.
I love the knobbly paper made from elephant dung. In Graskop on a side street just behind a bank (FNB) is the shop of the wildlife artist who sells lovely prints of animals and birds. From postcards to calendars.