Marvel at the size of Assumption Cathedral. Go to a mass if you can or simply wander around and appreciate the serenity.
You'll have no end of sightseeing choices when visiting Smolensk. Assumption Cathedral is a good place to begin here, but there are plenty of other interesting sights to check out. Museum of Russian Vodka, Museum Reserve City Forge of the XVII Century, and Smolensk Flax Museum are close to the principal church and will fill in your day nicely.