I was there and I did not need my passport.  Citizens with or without a passport are able to travel there safely.  You will not pass through any immigration check.  You might want to take the passport along, along with your drivers license.  You can rent a car there with the license.  Keep in mind that it is quite hot there.  Also May has the most rain of any month. Check out  weather.com for the latest weather. Temperatures are very constant.