Sop Ruak, a few kilometres north of Chiang Saen, is where most visitors get their Golden Triangle moment: the Mekong and Ruak rivers meeting, with Laos across the water and Myanmar close by. The viewpoint is touristy, but the geography is real and easy to grasp.
Do not make this the only reason to come to Chiang Saen. It works best after you have seen the old Lanna town, when the border landscape feels like part of a longer story rather than a signboard stop.