Bayfront Park, a Menlo Park city park, isn't really a mountain biking destination, but bikes are permitted on the dirt and gravel trails and it's only two miles from my work.
Bedwell Bayfront Park, located where Marsh Road intersects with Bayfront Expressway, is a popular lunchtime walking and jogging area for the workers at the nearby office parks. Leashed dogs are permitted on the trails here. In spite of some speculation about public safety, Menlo Park police report almost no instances of violent or property crime at the park.
Dirt and gravel travels crisscross the small hills of this park which was built on a closed landfill. You can see the methane recovery plant at the back of the park, along with the remains of a retired sewage treatment plant. A wide gravel road runs along the perimeter of the park, from where you can watch the shorebirds and other critters in the adjacent sloughs and mud flats.
No trail signposts point the way, but it's impossible to get lost in tiny Bayfront Park, which encompasses 160 acres. The perimeter road is only two and half miles, so if you somehow lose your way, either head to the top of a hill and look around, or go downhill until you hit the perimeter, then ride until you've reached the park entrance.
I've ridden about every trail at Bayfront on a fixed gear bike, so this isn't exactly challenging mountain bike terrain. For a quick, relaxing lunch time fix, though, the location works and the views of the Bay are pretty nice. The unmarked trails invite exploration. Any kind of bike is suitable and riders of just about any level can handle most of the park trails -- there are a couple of moderately steep sections but you can easily avoid them if you want.