Shanhaiguan pairs naturally with the beach resort of Beidaihe (25 km south), once the official summer retreat of the Communist Party leadership and still a working seaside town with grand old villas, kilometres of sand and a famous bird observatory in the coastal wetlands. The two together make an easy weekend.
Further afield, the Eastern Qing Tombs (Qing Dongling) — a UNESCO-listed complex of mausoleums of the Qing emperors — are about 4 hours' drive west toward Beijing, and could be combined with the trip back. The Chengde Mountain Resort, the Qing summer palace north of Beijing, is 6 hours west and worth its own overnight rather than a day-trip from Shanhaiguan.
The traditional Battle of Shanhai Pass site, just south of the town, is where the Manchu armies finally broke through the Ming defences in 1644 — an event that ended the Ming dynasty and inaugurated the 268-year Qing. There is no formal memorial but the local history museum has a good summary.