In general, Thai Buddhist temples (Wats) are open to everyone. The temples are very often not just beautiful but they also have a very pleasant, calm, soothing atmosphere and you should try to visit at least some of them. There are, however, several rules that you must observe. Modest clothing is required, which means for ladies: a skirt or dress that covers the knees and a top with sleeves that covers the shoulders and is not transparent. For men, modest clothing means long pants and a shirt that covers the shoulders. Shorts will not do!
Before entering any building in a temple you must always take off your shoes. Outside are often shelves for the shoes (sometimes separate shelves, one for Thais and one for foreigners). In general, you may photograph anything in a Wat, but taking pictures while people are praying is not polite! You should also never, ever have your picture taken with any Buddhist image. The monks may be photographed, by they often do not like it they just hide when they see a camera aimed at them. Women may talk to monks, but may not touch them (and the monks may also not receive anything directly from a woman's hand). You should also never, ever touch the head of a Buddha statue or point with a foot a it, or climb on anything in a temple.In general, just ask yourself: would I behave this way in a church or temple of my own confession? and act accordingly.
How Not to Offend
In general, Thais are friendly and polite, and you should try to act the same way. It is very bad behavior to start loud arguments or challenge someone in public. Harsh criticism, shouting, or other displays of anger or rage will not get you anywhere in Thailand. A person's head is considered sacred and it is an insult to touch it. The feet, on the other hand, are considered very impure, and you should never point at a person, or a religious object with a foot or step over somebody. The king of Thailand and royal family are held in genuine respect by the people and you should never speak derogatively about them or act disrespectfully with regard to them (for instance, never step on a Thai bank note - it depicts the head of the king. People have been known to be arrested for this!)