Udo sits just off Jeju's eastern tip, floating right in front of Seongsan Ilchulbong. Seen from above, the island's silhouette supposedly looks like a cow lying down — hence the name, which literally means 'Cow Island.' It's a quick 15-minute hop from Seongsan Harbor by ferry, with boats running frequently throughout the day, so it slots neatly into a half-day or full-day add-on to a Jeju trip without much scheduling hassle. The waters around it are gorgeous enough that the whole area is designated a marine provincial park — most people consider it a mandatory crossing on any real Jeju itinerary.
The beaches are the main event. Seobinbaeksa gets its name from 'hongjodangoe' — reddish algae nodules that wash ashore and pile up like coral fragments, giving the sand a strange, pale, almost bleached-coral look against startlingly emerald water; photographers tend to get stuck here for a while. On the other side of the island, Geommeolle Beach flips the script entirely with jet-black volcanic sand, a stark contrast that feels very on-brand for Jeju. Climb Udobong at the island's south end if you've got the legs for it — the summit hands you a full view of Udo plus Seongsan Ilchulbong across the water.
Getting around is part of the fun. Regular rental cars are largely restricted on the island (with some exceptions, like guests staying overnight), so most visitors rent an electric bike or electric three-wheeler near the harbor instead. The coastal ring road runs about 17km around the island, and cruising it slowly on a trike — wind in your face, stopping wherever the view demands it — is easily doable in half a day.
No trip to Udo is complete without peanuts. Locally grown peanuts show up everywhere as peanut ice cream, the island's signature snack, and peanut makgeolli makes a fun souvenir-slash-taste-test. Time the ferries right and the standard move is: morning boat over, a couple of beaches, a full loop by electric trike, evening boat back — easily done with just one day carved out of a broader Jeju itinerary.