Booger, just be careful driving and do not have an accident. The taxi drivers can be aggressive but I have driven a bunch over the years and never had any problem. I would not be obviously drinking and driving but if you stop some place for food and have a couple as long as you are capable of driving there should not be any problem.
If you were to get in any fender bender and the cops came don't try to speak any spanish unless you have a good command of the language (stick to "your" language) otherwise they will verbally abuse you so to speak.
You will have a great time and I forgot to mention to try the Cuban restaurant out of town a short ways. If you like ceviche it is really good there. Also if you want to see how the other half live go to Villa Rolandi (most expensive hotel on Isla) for a drink. They have a nice bar and good appetizers :-)
I think you will be content to say on the island but if you did want to do a day trip to Playa Del Carmen the cheapest transportation are the Collectivos or a bus.
Are you using a private transfer from the airport to the Ultramar ferry?