Beach, city and night
On the beach, swim shorts and a thin shirt are enough, plus sandals that dry fast. In Bangkok, wear a cotton tee, light trousers or knee-length shorts, and sneakers, because the city is hot and walking is long. The BTS and the malls are cold, so carry a thin overshirt. For nightlife, wear a dark shirt, light chinos, and closed shoes. Clubs and rooftop bars may refuse flip-flops. For island hopping, wear swim shorts, a rash guard, and a cap, and carry a dry bag. Night markets are casual and crowded, so keep your wallet in a front pocket. Pack light, since you can buy cheap tees in Bangkok.