Chiang Saen is an old Lanna town on the Mekong River, north of Chiang Rai and close to the point where Thailand, Laos, and Myanmar nearly meet. It is quieter than the Golden Triangle name suggests: riverfront restaurants, temple ruins, and a low old-city wall rather than border-town drama.
The reason to come is the archaeology. Chiang Saen was one of the oldest centres of the Lanna kingdom, and its brick chedis and ruined viharns still sit inside and just outside the old walls. Wat Chedi Luang gives the town its clearest landmark, Wat Pa Sak is the most graceful ruin, and the Chiang Saen National Museum helps make sense of what you are seeing.
Chiang Saen works well as a slow day from Chiang Rai, but it rewards staying overnight. Early and late light soften the brick ruins, the Mekong riverfront is easy to wander, and Sop Ruak's Golden Triangle viewpoint and the Hall of Opium are close enough for a half-day side trip.