Last month, my husband and I decided to hit the most crowded area of Yellowstone -- Old Faithful and the Upper Geyser Basin -- early in the morning. We got there before 7 am and were rewarded by O.F.'s eruption shortly after we got there. There were only 3-4 dozen people on the Boardwalk surrounding O.F. No tour buses had gotten on the road yet.
On our second day, we were at Gibbon Falls, and then Tower Falls, before 9 in the morning. Those are two other crowded areas.