Since Thailand is more of an adventure place anyways, dressing up meant being uber-casual and relaxed. In other words, don't try to overdress looking like Paris Hilton in Malibu Beach.
For those of you who would like to know what's an appropriate outfit to wear when going to the beach or just explore, here's what I wore on my first beach day in Phuket (remember Thailand is a contradictingly conservative country).
$UniQlo Fedora straw hat
$American Rag strapless sundress
$Havaianas flip flops
$Urban Outfitters combat utility backpack
REMEMBER: Keep it simple and comfortable!