If you want a very scenic and long route with a lot of adventure, from the Parque Enriquillo you can take a gua-gua to the boarder town of Jimani, there you can have a wonderful lunch and then hop on motorconchos and cross the border to the Haitian side. Once there you can pick up a haitian bus that will take you to Puerto Principe, it is about a 2 hour trip (due to poor road conditions). It will leave you off in the middle of the central market area.
If you choose this route to travel be sure to leave Sto. Domingo about 6 am or on one of the first buses out because you want to account for any delays and also want to aviod arriving at night into puerto principe.
I do not quite remember how much it cost but is was cheaper than Caribe Tours or Terra Bus by about half the price. If you take different buses and connect your way to Haiti it also allows you to stop and see some of the towns along the way and Lago Enriquillo as well.
I personally had a wonderful 4 day trip to Haiti full of adventure, and although we only saw a small percentage of the country it makes you appreciate more what Dominicans have in their country. It is an eye opening experience.
I can recommend some great places to stay, both cheap and not so cheap just email me if you would like more info.
Good Luck!