Packing for Thailand's rainy season requires some careful thought. You can still travel light as long as you have a good shopping list for your arrival.
Before You Leave for Thailand
Think waterproof, especially for your packet of critical items. Plane tickets, travelers checks and passport need to be doubly protected. Consider keeping each in a zip lock bag, then keeping the zip lock bags in a common waterproof pouch.Invest in a day pack that is as waterproof as possible.
Trekking? If you plan on taking any semi-serious treks, take shoes that will gripwell on slippery slopes. Don't trek in shorts. Your legs will suffer scratches from the flora. Trousers with lower legs that "zip off" can be a treat.
Think cotton, clothing should be easily washable and quick dry. Denims are great looking and rugged, but they dry very slowly. Cheap trainer type fabric or synthetic shoes will dry out more easily than leather.
Extra clothing. An extra shirt / t-shirt or so can be a welcome luxury. Wet shirts are enemies. If you don't want to carry them to Thailand, be sure to shop immediately on arrival in Bangkok.
Sun glasses But not too dark. Rain in your eyes on a bike ride or in an open vehicle is no fun.
Electronics, such as cel phone, pda, camera or computer (are you really sure you want to take one?) need to have wet weather protection. Get some silica gel packs and keep one or more in each bag.
Make a good shopping list for rainy season.
