My daughter filled the animal bingo card in about 8 hours at Yellowstone this week. Though the grizzly we saw was actually outside the park. Saw some huge Big horn sheep roadside which is usually more of a winter sight to see them so close.
I've been to the park about 50 times, and renew my claim that there is no better time to go than October.
Nobody's there, save some Asian tourist buses and a few Euros. Aspens are gorgeous, temps are perfect.