Mountain by the town of Mammoth, literally right at one of the entrances to Yellowstone.
Norris Basin! A definite must see!
Excelsior Geyser Crater, there is a bubbling geyser spot in there, but the steam usually hides it. For some fun facts,
read the wikipedia page here about this thing.
The Grand Prismatic Spring. A must-see. <3
Some of the scenery in this place. Pictures don't do it a bit of justice. You totally miss the depth and the breeze and the sensations of standing in these places.
Lastly, here's the Lower Falls, also known as the Grand Canyon of Yellowstone. <3 Again, these pictures are beautiful but can't compare. I would encourage people to visit Yellowstone sometime in their life. There's nowhere else like it in the world.