Since you are going in May, not all places will be open. I was also thinking about Vore Buffalo Jump which isn't too far from Devil's Tower, but it doesn't open up until summer.
I'm not sure if you will find enough to entertain you in Sturgis for a day/night since the rally isn't going on. From what I hear, the town is very quiet during non-rally times.
In addition to those places mentioned by Peach, I'd also try to include a drive through Spearfish Canyon, Badlands National Park, Crazy Horse, Wind Cave and Jewel Cave. A rather "quirky" place would be the Cosmos Mystery Area, but I'm not sure of the hours in May. The Cosmos is a place of unexplained happenings, like balls rolling uphill, etc.
I have read the Firehouse Restaurant is a good place in Rapid City for dining. You might also consider Big Time Pizza in Keystone (the BEST pizza you will find!), any of the lodges in Custer State Park, the Alpine Inn, and if you are into wine, I hear Prairie Berry Winery in Hill city has some good homemade wines.
Definitely order the travel planner Peach mentioned. It will be a tremendous help.
Have a great trip, and let us know if we can help further.