Air Santo Domingo is the only carrier providing service within the Dominican Republic; its small planes may rattle like East German cars, but they're reliable to a fault. Fares generally run a little higher than US$50 between most destinations. Use your international cell phone rental to make an airline reservation easily.
Buses are the preferred way to see Haiti. They're cheap, convenient and, if you use Metro Bus or Carib Tours, air-conditioned and comfortable. There are also gua-guas, smaller and less-comfortable buses that are favored by locals. They're a great way to scoot between locations in the same region, and fares rarely break US$2 in Dominican pesos.
Car rental in the DR is fairly expensive; plan to pay at least US$50/day for a mid-sized sedan, plus another US$10/day for insurance. You must be older than 25 and have a credit card, valid driver's license and nerves of steel. There are few road rules in the Dominican Republic and the few on the books are rarely enforced. Traffic police are notoriously corrupt; you may be stopped an 'fined' US$5 or US$10 for such offenses as 'driving a dirty car' or 'failing to break fast enough.' One LP writer was told simply, 'Give me money. I want to buy a beer.' It's best not to argue; tell the officer you're sorry, promise to never break the law again, and keep a small amount of money in your wallet for just such an emergency. Use your international mobile phone rentals to rent a car.
Renting a motorcycle might be a better idea, if you're comfortable driving on dirt roads. They're cheaper to rent - think US$16/day - and they'll get you to places neither buses nor cars can go. As an added bonus, traffic police are less likely to hassle you on two wheels.
There is a daily ferry service that links Samaná and Sabana del Mar. It's limited to people, bicycles and motorcycles, and saves you a four-hour trip by bus or car.