Yellowstone is more then big enough to spend a second (or even a third) day exploring. I probably would of even stayed longer if it wasn't getting so cold at night. I saw a lot of wildlife in the east park of the park, more buffalo, elk, and a family of big horn sheep.
After spending the day exploring the east and northern parts of the park, I decided to camp in warmer ground and made my way out the north exit into Montana.