Geography has treated Chiang Saen well. Perched on the bank of the mighty Mekong, it stands near the center of the infamous Golden Triangle. Likewise, Chiang Saen treated Thailand well by producing HM Mengrai, scion of the House of Chiang Saen, who proved a major architect of the Lanna Kingdom.
With much of the city gate intact and some of the walls standing, it's a short flight of fancy to imagine yourself in ancient Lanna.
While you might not want to try to cover the entire town on foot, Chiang Saen town is extremely walkable.
If you like museums with Buddhist artifacts and old photos, visit the National Museum. Besides being in the center of town, the setting is quite nice. And it's cheap.
Getting there is pretty painless. Grab a bus at the central Chiang Rai bus station, or catch one at any number of stops along the highway.
Motorcyclists will enjoy the 60 km ride on a rented bike, available from many Chiang Rai shops.
It's a short ride by bike or seelor to the prime tourist attraction in the area, the "Golden Triangle" marker. The area is commercially overdone with souvenir and handicraft stalls lining the roadside opposite the river.