Must sees are the Grand Canyon of Yellowstone, Old Faithful Geyser Basin, Norris Geyser Basin, Mammoth, Lake and definitely Lamar Valley that's where the wolves will be hanging. If you get a chance do the Tetons to the south and check out Chico Hot Springs just north of mammoth to relax. Also Cody Wyoming is awesome, the buffalo bill museum in Cody is great. If your in to fishing especially fly fishing it's awesome in the park. I've been a lot of places but Yellowstone and the surrounding areas are my favorite. Enjoy