Hiking up Hallasan via Seongpanak trail, absolutely beautiful, one of the most amazing trail I had been to. The peak is a volcano crater turned into a lake.
Took the 6.20am bus 781 from the terminal and reach Seongpanak in 30 minutes. This trail is popular, with many people hiking on a Monday morning with good support infrastructure. You can arrive earlier and have breakfast here. The journey to the peak is about 5h, and you could come down via another trail.
There is a shelter near the peak with toilet and sell hot coffee for 500won, amazing!
After hiking up Hallasan through Sangpanak, we hike down using Gwaneumsa trail. It's beautiful and different from Sangpanak, so it's worth to try this route going down. The infrastructure of this trail is less developed as compared to Sangpanak.
After coming down, take Buss 77 to reach the main road. After which, you need to switch buses to Jeju city or other places (Check Google Map for public transport navigation, the route and bus number is very accurate, but the timing is not.
Trying the Street food (there are less than 10 stalls).
My favourite is the fried stuff stall, where we tried a stick of variety stuff and crab shell with creamy fillings. Jeju orange drink (shave ice) is available for 3000won, it is okay. The Korean rice cake stall is more chaotic and crowded, but they do speak Chinese and English. Make sure you queue from the right side and make your orders (state eat here or take away, number of people, rice cake or variety of fried stuff; I think they have sushi and fried dumplings as well). They will seat you once space is available.