Charleston is a very walkable city, especially in the downtown area. However, if you plan on venturing out to Folly Beach, it's recommended to rent a car for convenience. Public transportation is also available via CARTA buses and the DASH Trolley. Expect to pay around $20 for a taxi ride from the airport to downtown Charleston and around $10-15 for each CARTA trip.
If you have extra time, consider visiting the Charleston Tea Plantation or touring the USS Yorktown at Patriots Point. For a shorter trip, skip Folly Beach and opt for a carriage tour instead. Additionally, if you're on a budget, there are plenty of free walking tours and attractions around Charleston to explore.