If you don't fancy the crowds at Badaling or the slopes at Simitai, why not try Huanghua. Lovely, a bit run down, I went on a foggy day in 2009 and it was deserted. Pay the cafe owner to let you onto the wall.
You can get a taxi all the way from Beijing, if you haggle, otherwise take a bus to Huairou and get a bus or taxi from there.
Google map: tinyurl.com/37gcezz
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