The Church of the Three Crosses is Imatra's major architectural sight, designed by Alvar Aalto for the Vuoksenniska district. From outside it is restrained and white; inside, the light, asymmetry, and three plain altar crosses do the work.
It is north of central Imatra, so it needs a deliberate trip rather than a casual stroll from the rapids. For anyone interested in Finnish modernism, it is worth that effort.