It's a steep one-hour climb, with more than 900 steps, to reach the top of scenic Mani-san (469m). At its summit you'll find Chamseongdan (참성단; 10am to 4pm), a large stone altar said to have been originally built and used by Dangun, the mythical first Korean.
Mani-san is 15km from Ganghwa-eup; take bus 3 or 7A (50 minutes). A taxi from Ganghwa-eup (₩20,000) takes half an hour .