We spent a month in Spearfish last summer. We used it as home base and drove to see a lot from there. Devils Tower is an easy day drive, visit, and drive back. In fact we drove from Spearfish to Belle Fourche SD (about 30 miles north) to the geographic center of the US and from there we took Hwy 34/24 to Devils Tower (only 55 miles away) a beautiful drive.
Also, I recommend Spearfish Canyon (Hwy alt-14 ) to Cheyenne Crossing. Be sure and eat lunch (or breakfast) at Cheyenne Crossing. It is awesome! That’s all that is at Cheyenne Crossing, as it is just a intersection.
Then stay with Hwy alt-14 into the town of Lead, stopping at the Sanford Lab visitor center where they describe the mining and now scientific research inside the 8000 foot mine. Very interesting. Traveling from Lead you will go through Deadwood before heading back to Spearfish.
Deadwood is very interesting too. Old town streets and buildings, Street shootouts, casinos, etc. Worth a day all by itself. We also drove from Spearfish to Custer, Badlands, Mt Rushmore, etc.