Bay seniors who think they’ve seen everything in their long lives, will be amazed to learn that one of the most popular tours in San Francisco is of our wastewater treatment plants. They occur one Saturday a month and last for two hours at the facility on the Great Highway. Lots of walking. Surprisingly they fill up early. Free. Other water sites worth a visit are the mini Greek temples at Pulgas on the Peninsula and another in Sunol. Really, Greek temples.