Definitely take the walk around Lake Merritt, the centerpiece of Oakland, and take your camera. About three miles around the lake. Parking at adjacent Lakeside Park is just $2 for two hours, plenty of time to walk the lake and take photos. Lots of water birds, and otters apparently have been spotted recently, too. Be sure to check the Garden at Lakeside Park, too! If you have more time, the Lake Chalet is a nice place to have a drink or enjoy a meal on the water.
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