Thailand’s National Theater predominantly shows Thai Classical drama and features performances of Khon (drama), where players wear traditional Thai masks. These are truly wonderful productions; extremely ornate costumes, excellent lighting and everything you could want. You can see exhibitions of Thai classical dancing and music on the last Friday and Saturday of each month.
The theater does, however, venture into more international aspects of the medium and it is worth getting hold of their schedule. Seeing a play at the National Theater might not be at the top of your priority list.