Buddhist, Chinese, and Muslim shrines are open to foreigners but since these are places of worship, please dress appropriately when visiting.  Shorts and sleeveless shirts are discouraged and you should remove your shoes before entering.  You may photograph monks (but women may not touch them), wats (temples), images (except the Emerald Buddha) and all Buddhist ceremonies.  A non-Muslim can enter a mosque except during prayer time; some areas are off-limits to women.  Ask first.  Hilltribe houses have special spirit rooms which are closed to outsiders; entering them will violate their sanctity.

Thais regard the feet as unclean.  Do not point them at others or step over those seated or lying on the floor.  Remove your shoes before entering a Thai home and wai (palms together at chin level in an attitude of prayer) to greet the host.  Thais are amenable to being photographed but if in doubt, ask first.