Nob Hill's heavyweight hood ornament is a must-visit for those intrigued by cathedrals and architecture. In addition to stunning stained glass, there are two meditation labyrinths (outside and in), and three massive organs.
The Episcopal congregation's historical roots go back to the California gold rush, and with a progressive approach to religion ("A house of prayer for all people, without exception") - including an "Interfaith AIDS Chapel" - the organization enjoys widespread local support.
A reliably good time to visit Grace Cathedral is during one of their (frequent) musical events - check the schedule. Tuesday night's yoga on the labyrinth is also quite popular :)
Address: 1100 California St, SF, CA
Website: http://www.gracecathedral.org/calendar/overview/musicevents/
Wikipedia: http://en.wikipedia.org/wiki/Grace_Cathedral,_San_Francisco
