The Sturgis motorcycle rally will be right toward the end of your trip in Sturgis, South Dakota.
If you love zillions of bikes and biker-people in one place, this is a "don't miss." But get lodging reservations now, or you won't find a bed for 100's of miles in any direction.
The rally lasts one week officially, but bikes will be arriving and lingering for two weeks on either side. Bob