If you're looking for outdoor adventure, make sure you visit Doi Sa Ngo. No easy stroll around the block, this hike is fantastic for some mid-vacation exercise.
You'll have no end of sightseeing choices in Chiang Saen. Doi Sa Ngo is a great place to kick off here, but there are lots of other worthwhile places to visit. Chiang Saen National Museum, Golden Triangle Park, and Hall of Opium are not far from the steep landmass and and will cap off your day nicely.