Recommended by locals and heavily marketed hotel in Okpo.
해금강
Designated "Korean National Scenic Site #2, this is a must-see location. Essentially just a locally famed island consisting of tall cliffs. However, the ferry cruise around the island will thread its way through the extremely thin cliffs leading into the island into a sea cave before backing out and departing through different route. Spectacular. Round-trip 3-hour cruises including both Haegeumgang and a 3½-hour stopover on Oedo from Jangseungpo.
외도
Typically included in cruises to Haegumgang, this is Korea's first privately owned island and host to the Oedo Botanica. It's somewhat of a cross between a botanical garden and one of East Asia's infamous "country-themed theme parks" and designed to make you think you're somewhere in the Mediterranean. Beautifully landscaped and covered with Greco-Roman statues. It's a bit prettier than the brochure describes as the gardens follow a tight valley surrounded by sheer cliffs. Take a detour to the bathroom as the off-the-path views from outside a couple of them are among the best on the island. Conceived by a couple that crossed the 38th parallel after the war quite literally penniless. Worth the trip and has a restaurant and coffee shop at the highest point that mostly only serves tea (Udon ₩4000, coffee ₩4000, tea ₩2500).