If You Have Only 1 Day
If you have only 1 day, you'll probably want to limit yourself to either the North or South Unit. Since it's more accessible from I-94, and more developed, it's likely you'll choose the South Unit.
If you're coming from the east, stop first at the Painted Canyon Overlook and Visitor Center to get a sweeping, panoramic view of the North Dakota badlands, then continue 7 miles west to Medora and the South Unit Visitor Center. Here you'll be able to view a film and listen to one or more ranger talks. There's also a museum with some of Theodore Roosevelt's personal effects. Be sure to stop in at the Maltese Cross cabin, behind the visitor center. Take the 36-mile scenic driving loop around the park, stopping at the overlooks (be sure to stop at one of the prairie-dog towns), and stop along the way to explore one of the scenic trails.