Fantastic experience: 20mins or so training on a segway, then about 2hrs travelling on it, seeing parts of SF you otherwise might not (away from centre). The Segway is a unique, very interesting and very easy to use mode of transport. The staff are very helpful and nice to be with. A bit pricey - 65 dollars each, plus you are expected to slip the tour guide 5 or 10 dollars.
Beach Street, near Hyde. 30 Bus from downtown (get off at North Point and Hyde), or take Powell Street cable car to end. www.sfelectrictour.com/