San Fancisco to Sturgis

For a most scenic route, I would head East through Yosemite National Park and across Nevada. Head for Southern Utah, and follow the 5 National Parks there towards Moab, Utah, through the Escalante Region. Even if you don't have much time for the parks themselves, the scenery all along there is awesome! Red Rock canyons, mountaints, etc. From Moab, go north to Interstate 70, then follow that East towards Denver, Colorado. From there, there are a few routes North to get to Sturgis. From Sturgis, it is a short trip West to Devils Tower in Wyoming, a lot of bikers go there during bike week. Maybe you could catch that on the way back. I see you have already been to Badlands and Custer State Park, but they might be worth another visit when you are in the area. For more details of all these places, see my National Parks Blog. Have a great trip!!