Depending on what part of Oregon you are aiming at, for the return trip take US 20 (as previously described, then also from the Ontario area across Oregon), or you could head north from the Black Hills on I-90 to Missoula, then across Lolo Pass on US12 (beautiful drive) to Lewiston.
If you take the latter route, you may want to allow a day to visit the Little Bighorn National Battlefield. Some very interesting tours there, a couple decent CGs nearby.
x2 on Devils Tower! And on Craters of the Moon if you use US20.
