Devil's Tower is a good waypoint before the Black Hills, but 1 night is enough. The visitor center exhibits are very good and including the short trail circling the base of the tower can be done in a half a day. The other couple of days can be used to advantage in the Black Hills. Consider going to Wind Cave NP and Jewel Cave NM for the very different tours if caves interest you. Also the ranger at Wind Cave told me a couple of years ago that their CG had never filled, so a likely place to get a spot without advance reservations. You can round out your day with a dip in the springs in Hot Springs, SD if you like. Enjoy a fabulous trip!