If headed over to go cave tubing, zip-lining, or exploring the Mayan ruins on the mainland, MAKE SURE to take long pants and a long sleeved shirt as well as comfortable walking shoes.
Other than that, it was shorts, t-shirts and swimsuits the entire time, my girl friends wore the occasional sun dress =) I don't think I even needed my sandals or shoes for most of my trip either.
We never went to Jaguar's or Big Daddy's - but there's something going on at one bar/restaurant or another every single night!
We have been zip lining, cave tubing, and exploring Maya ruins many times, and we just wear our shorts, tank tops, and water/hiking Teva sandals and have been just fine. We sometimes need bug spray, but that depends on the time of year.
I agree with others...just take a carry on. In the morning, we put on shorts, shirt, and sandals/flip flops and we are good for the entire day. Even going to dinner in the evenings. No reason to dress up ever. Maybe a sundress at the most. Everyone over packs their first time!