For those of us who don't own cars we either hire a car (for a trip or a day) or we rent. Avis is a reliable, though expensive, option for renting cars. Employees tend to be very well trained and friendly, and many speak English. You can rent a car and if you don't have a guide, the office can help you find one.
You could bust all the windows and drive your rental car back on four flat tires and Avis still wouldn't charge you extra. Seriously. This Avis location knows and understands Haiti; they get the fact that any car driven in Port-au-Prince is going to come back trashed. And when that happens, it won't be held against you.
If you are flying in they will pick you up at the airport (either domestic or international) but you need to make arrangements in advance.
Mon-Sa: 08:00-17:00
Su: 08:00-12:00