What should I bring on my Akumal trip?

A few swimsuits and sarongs, a couple pairs of shorts, a few tank tops and flip flops - you don't need much more in the way of clothes. It took me many years to learn not to overpack.
I still take quite a few things that most would consider unnecessary but I do like comfort and convenience. A few things that I always pack are:

A SPONGE - I like a nice clean one.

SMALL KITCHEN KNIFE - I just like mine!

A SMALL ROLL OF DUCT TAPE - because if you can't duct it, f* it

BAGGIES - the gallon size works for everything: it is very humid there and
if you open a bag of chips in the evening, by the next morning they probably
won't be crunchy anymore! I put all food that has been opened into baggies
to keep it fresh. Large baggies also work for wet clothing and just to stay organized.

MP3 PLAYER WITH SPEAKERS - it is so nice to be able to take your music
out on the patio or down to the beach with you.

INSULATED MUG - I like everything ice cold and without one of these I would be drinking warm beverages which would not make me very happy!

SOFT SIDED COOLER - A must if you enjoy cold beers and don't want to make a hundred trips to the condo. Also a must for day trips. Mine holds a 12 pack with ice and
is on wheels but folds flat and fits nicely in the bottom of my suitcase.

MY PILLOW - sounds crazy but I need my pillow (which is very thin) or I don't sleep
well at all. I put it in one of those space bags so it is really flat (about the size of a sweater).

FIRST AID KIT - a small one with a few extra items for marine stings and stuff.

SMALL FLASHLIGHT - for walking home at night. It is very dark and there are no street lights. If you are there during turtle season (May - September) please
use a red lens on your flashlight when you walk on the beach at night.
If you don't have one, just use a piece of red cellophane with a rubber band to hold it on.
The red filters the light so it doesn't disturb the mama turtles trying to nest.

SNORKELING EQUIPMENT--A tight-fitting mask is absolutely essential
if you want to enjoy your snorkeling experience! It is such a drag
when you're out there in the water and your mask is leaking. A purge
snorkel is also important so you don't swallow water.
A few years ago I found blow-up noodles at Wal-Mart - very handy
if you aren't a strong swimmer and easy to pack!

Those are the "extras" I like to bring. There are more but those
are the biggies for a great time in paradise!