There's something unnerving yet gripping about seeing places of the dead. If this seems interesting to you, take a look at Olleung.
Situated in Yangju, this tomb site is well worth including on your itinerary. But you'll find plenty more to see in this area. Bukhansan National Park, Majang Reservoir Shaking Bridge, and Uijeongbu Stadium aren't too far from Olleung.