The path around Lake Merritt is a pleasant hour-ish hike in downtown Oakland. In addition to a startling variety of waterfowl (apparently it’s the first official wildlife refuge in the US) Lake Merritt is the home of Fairyland which I fear I may be forced to visit in the not to distant future.
(3.76 miles, 1 hour 2 minutes, average 3.64 mph, view in Google Earth).
Hike starts at: 37.808922, -122.263224.