Don't rent a car if you don't need to... no point of driving when you'll be drinking and/or other things all day long.
My wife and I went a few years ago and did a 'Bob Jeep @ Chukka Cove' tour through www.chukkacarribean.com
... It was $125 for the two of us, and was basically all day.. they picked us up from the hotel in a zebra 110, then drove us around the island, brought us to some local places for food & shopping and then to Bob Marleys house/museum. If you go to Bob Marleys, ask for Captain Crazy for your tour guide, you won't regret it.