On the banks of the Mekhong River at the southern-most point of the Golden Triangle tourist area is the ancient city of Chiang Saen.
Chiang Saen today is an interesting place to spend some time, although most visit on a day trip from Chiang Rai or Chiang Mai. You can easily make out the remains of most of the city's walls and moat, as well as several temples and defences. In the grounds of the old Wat Mahathat is an information center that can provide you with current information about what has been restored.
(http://thailandforvisitors.com/north/chiangrai/chiangsaen/)