If you don't want to go to the Rally, skip the Black Hills and spend your time in Colorado. Ride the whole damn state west of I-25. The most beautiful and best motorcycle riding country in the US. If you want to see the Grand Canyon, the best ride is on the North Rim. From Amarillo I would head west instead of north and go to Santa Fe or Taos then to Pagosa Springs or Durango or Cortez and start from there.

No matter where you go you are going to enjoy it but the ride from Texas to SD is barely bearable if you go directly north through OK, KS, and NE. Flat, straight, boring and hot all the way. Good luck and ride safe.