Shankh Monastery lies outside Kharkhorin, far quieter than Erdene Zuu but worth the detour for travelers trying to understand Mongolia's Buddhist history beyond the headline monument. The monastery is associated with Zanabazar, the great 17th-century religious leader and artist, and has been revived after the damage done to religious institutions in the 20th century.
The appeal is partly the silence: low buildings, open steppe, and a working religious atmosphere rather than a showpiece museum. You need transport from Kharkhorin, and it is best combined with Erdene Zuu and the Orkhon Valley landscape rather than treated as a separate day.