I’ve been riding at Bayfront Park in Redwood City, CA. The park is right on the San Francisco Bay. Views are great. It has un-kept fire roads around the perimeter (~1 mile) and walking trails with some single track trails all around the inside.
This has been a fantastic place for me to learn how to unicycle XC and muni (although saying these trails are muni is pushing it). For a beginner, the trails are very challenging. The hills are only 20 to ~70 feet tall. Some are steep enough that I can’t make it up even after three months of trying, and wish that I had a brake for the ride down (bad knees).
I would highly recommend this park to anyone who is new to unicycling and wants to try some “off road” trails in the middle of the Bay Area.
Here is a website with more info: http://www.bahiker.com/southbayhikes/bayfrontpark.html