Sometimes I look at the need to go somewhere a distance from where I am staying (like I do from Accompong Town to almost anywhere!), I use some creative thinking to see and do something different.
You might want to look at taking the http://www.knutsfordexpress.com/ from Negril to Falmouth (2 adults for a little over $60 US Round Trip) and then stay overnight at the http://www.glisteningwaters.com/hotel.html (don't know the exact price but worth checking into). The bus from Negril leaves at 7:15am and you would leave Falmouth at 9:30am the following day.
You might be able to do this trip for about the same price as taking a Private Red Plate Taxi and probably just a little more than renting a car without all the hassles driving on the sometimes kind of crazy highways.
Just an idea.
Peace and Guidance