Billings is a car city. Public transit exists (Billings MET Transit runs several bus routes) but is slow and not practical for sightseeing. Most of the main attractions outside the immediate downtown core — Pictograph Caves, ZooMontana, Chief Plenty Coups State Park, the Rimrocks — require a vehicle.
Downtown Billings is walkable and compact. Montana Avenue, the main historic street, is about a mile long and easily covered on foot. Taxis and rideshare services (Uber and Lyft) operate in town if you need a lift from the airport or between downtown spots.
Car rental is available at Billings Logan International Airport from all the major companies.