We are going in September to Yellowstone. Since the park lodging is probably booked up, Cody, WY may be a good place to stay if you want to on the east side of the part. You can also ride Chief Joseph Scenic Byway and Bear Tooth up to Red Lodge, MT. Walk the geyser basin where Old Faithful is. Around Mammoth, you will probably see a herd of Elk. When we stayed there a couple of years ago in Sept.. there was a huge male elk and with a large group of females/youngsters. Apparently they stay in the Mammoth area since the grass there is waters and predators don't come into town. Buffalo seem to be in the east and northeast part of the park. The Canyon area is very pretty and the waterfall there is quite stunning.
As for Tetons, go up signal mountain for a nice view. Moose Junction, at the river there.. you may see a moose.. they apparently like that area.
I hope that helps. The national park website also has decent detail on the park and various activities that go on there.
Also check motorcycleroads.com for the wyoming roads to ride. Enjoy your trip.. can't wait for ours

Hollyday