The island is a hiker's paradise, with numerous trails, but beware that some of the paths can be rough going, and it is very easy to get lost. You will certainly need a guide for any treks up to the volcanoes.
If you want to travel around the island independently, you'll have to get a 4WD. Hotel Ometepetl rents old Toyotas and Suzukis for approximately C880 for 12 hours. Taxis are plentiful, and drivers will swarm you as soon as you step off the boat, as will touts selling rooms and tours.
A pickup taxi from the port to Santo Domingo beach should cost approximately C360.
Old-school buses lumber around the island at hourly intervals between Moyogalpa and Altagracia, and they usually travel in a counterclockwise direction. The fare for these buses is C6. All buses leave directly from the Moyogalpa dock. The journey between the two towns takes 1 hour, and from Altagracia to Balgue, it takes another hour. There are less frequent services to outlying towns like San Ramón and Merida. The bus service on Sunday is much reduced and even sometimes nonexistent. Be warned, these bus journeys can be slow and uncomfortable, and if you are traveling across the isthmus, they can seem to take forever.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.