Jeju is a volcanic island off the south coast of Korea, and it has a landscape unlike the mainland, with a central mountain, black basalt shores, lava caves, coastal paths, orchards and fields of flowers. Korea Trip Map covers it as a single region, with planners that group sights by part of the island. It is a popular getaway for Koreans as well as for international visitors, and the local food, including black pork and seafood, is part of the appeal.
Getting there is easy. Flights from Seoul take about an hour and run frequently, though seats fill up during holidays and summer, so book early. Once on the island, the main question is how you will get around. Sights are spread out, and a rental car gives the most freedom, but you should check ahead whether your license or an International Driving Permit is accepted. Buses and taxis work for shorter plans, with more time spent traveling between places.
Weather matters more here than on the mainland. Wind and rain can change plans quickly, typhoons can disrupt flights in late summer and early autumn, and winter on the coast is windy. Spring brings flowers, summer brings beaches, and autumn is good for walking. Plan at least two full days, and keep a flexible schedule. Prices, hours and transport options change, so verify them before you travel.