If you going to costa rica and plan to travel and stay in several different places i recommend hiring a driver. We had considered renting a car but were glad we did not as driving in costa rica can be tough and stressful. Certain roads close sometimes (the new highway was closed because part of the road collapsed) and we wouldn't have known that. Also, there are very few signs and it gets dark around 5 pm. It was also very foggy with narrow curvy streets in many areas.
We hired Leonardo "Leo" Rodriguez to take us from san jose to arenal. then arenal to jaco. Jaco to manuel antonio and finally from manuel antonio to san jose (over the course of a week). He was very informative, always on time, and gave us a better rate than several others i got quotes from. His email is email@example.com and his phone number is 506-8378-0001 .
He also does tours, hiking, and many other things if you are interested. He is highly recommended. This option may be slightly may expense than renting a car but was by far a better option for my wife and I.