Udo — "Cow Island," supposedly shaped like a lying cow — sits 15 minutes by ferry off Jeju's east coast, and its 17 km coastal loop is the best easy bike ride in Korea. Ferry, bike rental, and the mandatory peanut ice cream run about 25,000 KRW total.
The Ferry
Ferries leave from Seongsan Port (the same town as Ilchulbong — combine them in one day) every 30 minutes, about 10,500 KRW round trip. Bring your passport or ID — it is required for the passenger manifest. The crossing takes 15 minutes and the boat is usually followed by seagulls being fed shrimp crackers by passengers, which is its own entertainment.
The Bike
Rental shops cluster at both Udo ports (Cheonjin and Haumok-dong). A standard bike runs 10,000–15,000 KRW for three hours; e-bikes and the little two-seat electric scooters run more and need an international driving permit for the scooters. Take the e-bike if there is any wind — Udo is flat but exposed, and the headwind on the north shore is real.
The Loop
Ride counterclockwise from Cheonjin port. The highlights in order: Geommeolle Beach, the black-sand cove with the sea cave at its base; Udobong Peak, the island's high point with a lighthouse and the 360-degree view back to Ilchulbong; then the long north shore past Haenyeo diving spots to Hagosudong Beach, the white-sand swimming beach on the west side; and finally Seobin Baeksa, the coral-sand beach that glows almost tropical blue on a sunny day.
The Food
Udo grows peanuts, and peanut ice cream is the island's religion — every cafe sells it, the ones near Geommeolle and Hagosudong are the institutions, and it is genuinely good: soft-serve with crushed roasted peanuts. For lunch, the haenyeo restaurants do seafood ramen and abalone rice. Budget three to four hours for the loop with stops, and check the last ferry time (usually around 17:00–18:30 seasonally) — missing it means an unplanned night on the island.