Go to walmart and get a 3-pack of the old-man style undershirts. (we call them wife-beaters). Then cut them off about mid-waist. You'll wear these a bunch - over swim suit with shorts, under big loose tank, etc.
Sunscreen in small containers to toss in purse or backpack. Even if you 'lather up' pre-sun, these come in very handy. Especially when driving around in golf cart when you don't think you'll need it - back of neck and tops of legs & feet can get scorched!
Lip balm with SPF
Immodium AD - essential. We've eaten from every street vendor there and never had a problem but the change in cuisine can be bad for some people. I keep one in my pocket at all times. Was a life saver one night at BC's (was night of arrival, hadn't even eaten there yet - must have been Houston Airport). 'nuff said
bras / underwear. Go commando - you're on vacation and is definitely breezier and more comfortable. Besides, how often do 'the girls' get a chance to breathe and be free - they're on vacation too, you know.
Any shoes with heels, are not 'sand-proof' or irreplaceable. Heels get mired in the sand, they'll be covered in sand, you'll prolly kick them off and get back to room and look down and say 'oh my, where are my shoes'? (happened. hehe)
Some say the glass is half full; some say it's half empty. I say "you going to finish that?"