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.

I drove through this valley several times. There are a lot of buffalo. I was trapped in the herd once in my car which is a real highlight. I had the window down and could hear all their noises as they moved along. I also saw elk once and a coyote. It’s crowded when the buffalo are near the road and also a lot of fun.

We entered the park at the East Entrance on route to Gardiner for an over-night stop. Buffalo herds prevented any thoughts of a relatively quick journey north with time to stop at Mammoth! Rangers were present directing traffic but buffalo have right of way and despite all the warnings about keeping your distance from wildlife allowing young children out of the car when a herd is clearly in the process of crossing is the height of stupidity. Even if they are not that near to your car it is suprising how the odd stray can suddenly appear leaving you in no doubt how big they are and how quickly they can move. It is wonderful to see them and better still if you can get out early before the crowds when you may have the opportunity to pull over and watch and better still photograph them from a safe distance.