Do not convert more money than you plan on spending, as most currencies in SE Asia are totally worthless once you leave the country. Credit cards are readily accepted in Thailand, but you will have trouble using them elsewhere in SE Asia except at major hotels and the most expensive shops. Travelers checks are good to carry to convert to local currency, but do not expect to be able to change them into US dollars without great difficulty and considerable expense. To carry cash, it is best to have a money pouch which you can wear under your clothing. It is also a good idea to carry your passport that way.
Pickpockets are a problem in the larger cities.
Bring stay-press clothing which washes easily and dries fast. No formal wear is required for tourists. Most of the year in most places the weather will be warm to hot. Bring shorts, but avoid very short shorts. Bring a hat which shades your face and a small umbrella or rain gear.