It'll take you several days to see all the sights outside of Beijing, and visits to the various sites require separate, often day-long excursions. If you only have time to see one site, you’ll want to go to the Great Wall; if you go the section at Badaling, you can also work the nearby 13 Ming Tombs into one outing. If you want to treat yourself to seeing several sites in one outing, and get an eye full of temples, head west, where you can make stops at the Fahai Temple, about an hour's drive from the center, as well as the tomb of Tian Yu, and the Jietai and then the Tanzhe Temples, all in one day if you’re touring by car. Another rewarding day’s outing takes you east to Zunhua, and the elaborate Qing Tombs complex. Wear walking shoes and bring a lunch to enjoy the surrounding countryside.
If you have time and inclination to go farther afield, consider a four-hour train trip and an overnight in Chengde, where in the 18th century Qing Dynasty Emperor Kangxi established a summer retreat of temples, gardens, and monasteries. If you’re looking for a little summertime relaxation, take a day and a night (or, for real relaxation, two days) at Beidaihe, to bask on the beach, chow down on seafood, or see where the Great Wall meets the sea at Shanghaiguan.