Many people choose to use a personal vehicle, but it is not necessary. All of downtown Rapid City's attractions are walkable.
To reach other places, such as the Rushmore Mall, one can use the Rapid Ride System (http://www.rcgov.com/pubw...) from the Milo Barber Transit Center. There are four loop routes featuring buses going in both directions about every 70 min around each loop. Fare is $1 and includes a transfer.
If you want to travel before 6 a.m. or after 6 p.m., you will have to take a taxi, there are plenty in Rapid City for your needs. Also, there are several travel services that are available for the "night life" to ensure a safe and inexpensive ride home, Scooters and Jonesy's Party Bus.