X3 on what other posters have said. My suggestion would be to take exit 131 from I90 and make a drive through of Badlands National Park. The other end of the Badlands loop road takes you right to the little town of Wall, where you can spend an hour or two at Wall Drug.
When you leave the Black Hills after your 1-1/2 days (lots more there than just the Mt. Rushmore monument) you can make the side trip off I90 to Devil's Tower for an hour or so, longer if you want to hike the one mile trail around the base of the tower.
Spend your time sightseeing, not just driving back and forth.
