Getting around the island isn't the easiest thing to do without your own transportation. Colectivos run hourly along the road in both directions; it's COP$2500 for a ride of any distance. As there are such big gaps between the buses, locals will often stop and offer you a ride.
Vehicles congregate at the airport waiting for incoming flights and ask for a flat fare of COP$23,000 for any distance. To avoid overpaying, walk a bit further from the airport and wave down a colectivo or pickup truck passing along the road; there's a standard COP$2500 fare. This might not be the best solution if you are carrying lots of bags, though. Aguadulce and Bahía Suroeste are a 15-minute ride by colectivo from the airport.
The best way to get about is to hire a scooter (COP$70,000 per day). This can be done at Moto Rent Airport, a house next to the airport (turn left onto the main road and it's the second house on your left), as well as at Hotel El Pirata Morgan in Aguadulce and at Cabañas Miss Mary in Bahía Suroeste. You can also hire golf carts (COP$180,000 per day) at Providencia Tours in Aguadulce. The only gas station on the island is on the outskirts of Santa Isabel.