HYDRATION/SNACKS: There is reportedly a source of mountain water half way up but personally I did not see it near the sign, and staff at one of the shelters advised me that no water is available. Hence suggest that you bring at least 3-4 bottles of water or isotonic drink in your day pack per person (maybe 1.5 to 2 litres), and be sure to conserve it wisely because you'll need some on the way down. I encountered some people on the way up who had no water left. Muesli bars, chocolate and fruit are good sources of energy. As well as fruit, Korean nationals tend to pack rice & meat lunches and have communal picnics at the summit.
WEATHER: Note that if you have the flexibility of several days from which to select, review the weather in advance and aim to choose the best day.....otherwise for example the clouds may restrict or entirely eliminate any views when you emerge from the treeline.
WALKING STICK: Recommended for safety walking in particualr on the volcanic rocks.
DAYPACK: Bring a small daypack to carry water, snacks, clothes etc and to keep your hands free. Even in summer bring a light jacket or sweater because it can get cool at the top with the wind and elevation. In Winter I'm sure it's really cold at the summit.
PROTECTION: Bring a hat for the higher elevations (when not in the forest), sun cream and lip balm.