Alii Drive is walkable end to end if you have the time — a slow three-mile coast walk. Bus service in town is minimal (the Hele-On Bus runs a single Kona route a few times a day and is more useful for residents than visitors), so most travellers rent a car at the airport.
The Big Island is large — driving the full coastal road takes about six hours. From Kailua-Kona, allow 90 minutes to Hilo (cross-island via Saddle Road), 45 minutes north to Hapuna Beach, two hours south to Volcanoes National Park, and three hours to Hawi at the northern tip. Petrol is significantly more expensive than on the mainland. Several operators on the pier rent mopeds and electric bikes for getting around town itself.