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.
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!