chengde (best to spend the night, methinks)
jiuhuashan
qinghuangdao
xiangshan (still in bj city limits, but takes the better portion of a day to climb and enjoy)
goose and duck ranch (huairou)

also google the beijing hikers group (not sure if that's the exact name); most of their hikes are probably too challenging for 5-year olds, but they have some easier ones also