[ma-skis.com]

ride a bike in beijing
go to the great wall that is old, i think simitai, where you do the hike, it's much more memorable than walking around the refurbished sections
eat everything, (try the street vendors, not the ones that look like a franchise on beijing, but the little side ones where old ladies are selling everything and anything)
bargain at 1/10th of what they offer.
walk through the hutongs

edit: 7-10 days in beijing is good if you want to do everything and walk around and enjoy the city on it's own.