St. Mary's Square is great place to visit if you're downtown. Any green space in this area is very valuable to the locals! This place is really gorgeous though. There are lots of trees and bushes, plenty of benches to relax on after a ton of walking, and a play structure if you have kids! There are restrooms available too.
St. Mary's Square is owned by the San Francisco Recreation and Park Department.
Additional Info
There is a parking garage below the park.
Muni access:
1-California - Sacramento and Kearny; walk south to park.
8X/8AX/8BX Bayshore Express lines - California and Kearny stop.
California Street Cable Car - California and Kearny or California and Grant stops.