All of the suggestions are spot on. After 2 trips to the rally, I would say riding the Black Hills is much better and important than being in Sturgis. One or two days in Sturgis is enough. I thought I was going for the Rally, but was blown away by the Black HIlls. If I go again, I may go in early September wen it is cooler and no one is there. Also, be careful in Sturgis: police are everywhere. Do not drink (to excess) and ride. Two of my friends had problems (which they created). I had to bail one out. The police are just trying to keep people safe, but you don't want to ruin your trip by getting locked up for an evening.
The ride to Devils Tower was nice, but the focus should be on the Black Hills; also, there is a huge rally at the HD dealer in Rapid City.