Kotokuji is a small temple just off the preserved street, reached by a short rise from the main village lane. It is not a grand temple, and that is part of its value: after the careful staging of Tsumago-juku, the temple gives you a quieter corner and a better sense of the village as a real settlement.
Stop here while wandering rather than treating it as a separate target. The short detour breaks up the museums and shopfronts nicely.