Within the first ten minutes in the park we spotted some buffalo (buffalo are actually called bison). They were chillin' on a patch of grass in the middle of the small park town of Mammoth, with cars buzzing around everywhere. They own the park and really just do what they want-walk on lawns, stop traffic, chase people.
I did not get closer than what the picture shows; they are intimidating beasts and can run at up to 40 mph. I also heard a lot of stories from my friends who had worked in Yellowstone about people getting too close to Buffalo and getting chased (my friend also petted a bison once). Rangers will also yell at you and can even ticket you, which it turns out is pretty serious, as any crime on Federal property is a felony.
In another spot int he park we saw a momma bison with a newborn calf. The afterbirth was still hanging out of the mother, so the calf, which they call red dogs for their color, was no more than 24 hours old.
From here we continued through the park and saw some sweet geysers, including Old Faithful. Another nearby geyser called Beehive actually showed it up, but it was still sweet. Japanese tourists were everywhere and were freaking out when the geysers went off. They all tried to take each others pictures in the short window of the eruption and pointed and yelled. Great sideshow.
The lodge by Old Faithful was sweet too. It is the original one built in 1901, and has some sweeeet woodwork.