It depends on how much time you have. If it's a short stay, you must of course see Old Faithful as well as Fountain Paint Pot and Midway Geyser Basin. The Grand Canyon of Yellowstone should also be seen - it's breathtaking. If you have some extra time, go for a swim in the Firehole River. The wildlife you will see while going from point to point is amazing. My husband and I have been there twice, and have seen Grizzly Bears, Brown Bears, Moose, Elk, Deer, and of course lots of Bison.