Leave Deadwood in the morning, connect to I-90 near Spearfish and choose either the Sundance approach or the Belle Fourche–Hulett route into Devils Tower.
Choose the approach by what you want to add
The Sundance route keeps you near I-90 longer and makes the Crook County Museum or Vore Buffalo Jump easy additions. The Belle Fourche route carries you through Aladdin and Hulett for a more rural approach.
Do not try to drive both plus Spearfish Canyon and still call the day relaxed. Choose one path out and another back only if daylight and energy support it.
Arrive before the busiest period
A reasonably early start from Deadwood can place you at the monument before the main summer parking pressure. Walk Tower Trail before lunch when possible.
If you arrive at midday, slow down. Use the entrance line as a signal to shorten the later itinerary rather than rushing the park.
Use Hulett or Sundance for the pause
Hulett is close to the park and fits the northern route. Sundance provides a wider set of services and the most direct I-90 connection for the return.
Confirm restaurant and museum schedules, particularly outside summer.
Return before the day turns into transportation
Deadwood offers plenty to do in the evening, so there is no need to force another Black Hills attraction into the drive back.
If you want Spearfish Canyon, Mount Rushmore or Custer State Park, build a second day.
Before you drive
Check current park alerts, highway conditions, weather and seasonal business hours. Download the route before leaving reliable service.