Nong Khai Freshwater Aquarium is a useful indoor stop for understanding the Mekong as more than a scenic riverfront. Its tanks focus on freshwater species, including fish associated with the river system that shapes life along Thailand's northeastern border.
It is a modest attraction, but a good one for families, hot afternoons, or travelers with an interest in local ecology. Combine it with the river promenade and Sala Kaew Ku rather than making a special trip from far away.