Cape Soya is the symbolic end of Japan. The northernmost-point monument is not beautiful, but the setting does the work: open sea, hard wind, low hills, and Sakhalin somewhere beyond the strait.
The cape has several nearby memorials, including a statue of explorer Mamiya Rinzo and an old naval watchtower from the Russo-Japanese War period. It is a geography stop more than an architectural one, but geography is exactly why you came this far.