Hayden Valley: It is Beautiful Valley and one of the best places to see wildlife. You will probably see herds of bison. .
Be prepared to get stuck in the traffic jam because buffalo/bison crossing the road. Many time bison was literally inches away from the car!!
This is Super COOL! They are majestic & massive!
Stay in the car though and be safe.
The gorgeous Yellowstone river along with the valley will provide amazing picture opportunity at various vantage point and lookouts.