A memorable part of visiting Jeju was meeting so many friendly visitors, mostly local, on ther ferry from Mokpo to Cgheju-do. But I did visit some of Jeju's volcanic attractions.
one day I took a bus from Jeju-do up to Seongpapak on the side of Mount Hallasan and from there gained some views of the summit, as well as visiting the WHS museum there.
on another day I took a different bus heading to Seongsan Ilchubong and got off near the Manjanggul lava tubes and went for a 2km hike down through a tube, a black tunnel big enough to run a train through. Along the way were some beautifully illuminated lava formations. The 1 km open for visits is only a fraction of what is there.
Returning to the bus stop I caught another bus to Seongsan Ilchubong, clearly a place of pilgimage for the hundreds of people gathered there to climb to the summit.