Rey's main attraction is this shrine built for a 9th-century descendant of Imam Hossein. The mausoleum has elaborate tilework; a golden dome; a pool in the courtyard; a 14th-century sarcophagus with intricate carvings, constructed from betel wood; and enough mirror tiles to make you dizzy. In the same complex is a shrine to Imam Hamzeh (brother of Imam Reza). Women need to wear a chador, available at the entrance.
Part of the complex is a small museum where you can see models of how the complex has evolved over the centuries and old photos of what the place looked like back in the 19th century.
Also leave some time to look around the adjacent Rey Bazaar, which is lively, compact and architecturally attractive.