Qazvin's Jameh Mosque is one of the city's oldest monuments, a congregational mosque that grew through several dynasties rather than arriving as one clean design. Its heavy brick piers and Seljuk details give it a different mood from the tiled showpieces of later Safavid Iran.
The mosque sits near the old commercial core, so it works best as part of a slow walk through the bazaar area. Dress conservatively and avoid prayer times if you want to look around quietly.