Climb the Cathedral South Tower — 533 steps to the Rhine panorama
Cologne's two-spired UNESCO Gothic cathedral is the first thing you see from the dock and the one sight no first-timer should skip. The interior (with the gilded Shrine of the Three Kings, the largest reliquary in the Western world) is free; the rite-of-passage is the 533-step spiral climb up the South Tower, squeezing past the colossal St. Peter's Bell — the largest free-swinging church bell on earth — to a platform staring straight down the Rhine. The Dom and the cruise landing stage are a ~5-minute walk apart, so it slots into any port window.