The town of Chiang Saen is a small quiet village where it's nice to spend a day or two, in order to have time to visit the town and its surroundings. I particularly liked seeing Wat Pa Sak, the "Temple of the Teak Forest," and its beautiful sculpted tower surrounded by trees.
To change your perspective on the region, I suggest you go up to Wat Phra That Pha Ngao, located4 km south of the town. Not only is the temple itself worth the side trip, but the view of the Mekong and Laos is amazing!
Finally, during your stay in northern Thailand, after admiring the Golden Triangle's landscapes, why not visit the Hall of Opium? This huge museum built in the side of a mountain presents in a very interesting way the ins and outs of poppy cultivation from 5000 years ago through today.