I was over at the hospital yesterday, visiting a friend, and there was a nice woman in the ER with chest pains. The doctor was recommending that she get to a lower altitude as a soon as possible. She had done nothing more than go for a hundred yard walk by the shore of Jenny Lake, but the altitude of almost seven thousand feet had taxed her heart. Second, drive very, very carefully at night. The animals which are such photo opportunities during the day will often move to the verges of the roads and highways to graze the long grass which grows there.
Daylight means a lot of traffic and they stay away from the roadways.
But in the night the traffic lessens and they aren't as wary of the occasional set of headlights as they are of the flood of vehicles whizzing along during the day. Hitting a buffalo in a Honda will ruin your holiday, believe me. You may not even survive it. Third, bring warm clothes. As I write this, the radio is warning that it will most likely drop six to eight inches of snow in Yellowstone Park tonight.