There is a zipline about 200 metres west of the back gate at the top White Pagoda Hill Park (白塔山). There are photos below, and a promo video on Tudou (which is pretty good, if you can ignore the 'busy' webpage). You can also do a zipline at Xinglong Shan (no photos).
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The White Pagoda Hill Zipline. It's 'safe', 'thrilling' and 'exciting'
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The plaque on the wall is the national 'amusement devices' safety certificate.
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You can see the White Pagoda, just behind the start of the zipline.