Ride the San Francisco Cable Car System from the turntable over Nob Hill down to either Bay Street or the aquatic park at Fisherman's Wharf.
The Powell Mason and Powell Hyde lines end up in different areas of Fisherman's Wharf, and it's certainly worth acquainting yourselves with the color coded signs and other such clues which will help you navigate the system.
The California Street line is much simpler and everything on this route runs the same way so the signs are on the cars themselves.
It's worth noting for older kids that you are allowed to ride the exterior of the car holding onto the traditional poles provided for this purpose, this is welcome but people riding this way must pay attention to the gripman as he will forewarn you of anything hazardous to your position.
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Cable cars run seven days a week with special schedules on weekends. Timetabled.
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San Francisco Cable Car