Thailand has pretty hot summers throughout and basic sun-protection becomes a must. Sun-screen, goggles, hats, wet-tissues, light clothes, umbrellas and plenty of water bottles should help you survive the heat. Mosquito-repellent crème/spray/jell become a real necessity as you travel further deep into dense areas like jungles and treks.
Good-grip shoes are a must if you plan on hiking or trekking, but carry only light sandals if you just want to roam around the city.Bermudas or shorts and t-shirts are perfect attire for shopping.
It is advisable to leave your luggage and valuables back in the hotel room or at other luggage facilities when you go out for sight-seeing or shopping. A light backpack will not tire you in hot weathers.
A digital camera, preferably water-proof comes handy when you explore the beauty of the beaches and temples.
Swimming costumes/trunks/bikinis are needed if you want to get wet in water.