The town of Jacmel is easily reached via one of the best maintained highways in Haiti and although the road can become rather narrow and twisty in certain areas, it is arguably one of the prettiest drives in the country in which to admire the countryside. Taxis can do the 3hr drive from Port-au-Prince at a fixed rate. Another (much less favorable) option would be to take a very crowded public bus, which is not for the fainthearted or those seeking comfort as they're always packed.
Jacmel is not a spread-out town and everything can easily be seen on foot.
The town is very walkable and pleasant, and due to its facing the Caribbean, the sea breezes make it quite enjoyable (much more than most inland towns in Haiti). The town is great visually and photographically and also has some of the friendliest people in the country. Small local craft stores abound and it's one of the best places to buy Haitian handicrafts and souvenirs.
Many buildings date from the early nineteenth century; the town has been tentatively accepted as a World Heritage Site and UNESCO reports that it sustained damage in the January 2010 earthquake.
Carnaval Jacmel's Carnaval celebrations take place a week and a half before the national Carnaval (ie. Mardi Gras) in either late January or early February, depending on the date of Ash Wednesday. It is the most famous Carnaval celebration in Haiti, and arguably second only to Trinidad in the Caribbean. The elaborate papier mache floats, flamboyant dancers, and large trucks turned into rolling stages on which famous Haitian artists perform make it a truly magical event. This is Jacmel's biggest weekend of the entire year, so make travel arrangements well in advance.
Jacmel is the sort of place that is wonderful to spend a laid-back day wandering the streets and enjoying a slow pace (which is a very rare trait in Haiti), and the ideal day would include a couple hours walking the city aimlessly taking in the old architecture, followed by a nice seafood or Creole meal and a leisurely swim at the beach located on the Hotel Cyvadier grounds -- which is open to the public but very hard to find for those not in-the-know (ask a local or a cab driver to point the way).
The promenade has been recently renovated with funding from Venezuela, and is a beautiful place to go for a stroll at any time of the day. Sometimes at night, Ra-ra bands play by the sea.
Excellent craft/souvenir shops are easy to spot while walking around the town. Do bargain (playfully but firm in what you believe is a fair cost) as prices will be inflated for Westerners.
Cayes Jamcel The beaches on Cayes Jacmel, just east of the city, are nicer (i.e. more sand than rocks) than the one in town, and there are places to rent surfboards or just string up a hammock.