Headlamp is mandatory.
Enough gear to survive temps ranging from hot to freezing. Rain gear in case it rains (thank God it didn't because I just had 100yen ponchos from the daiso).
We overpacked snacks of course. Bring shitloads of water or prepare to pay shitloads for it. Save water for the descent because there is nowhere to buy it on the down trail and you will get super dehydrated like me. If it's a weekend or holiday, prepare for it to be ridiculously crowded with thousands of people. Look out for people altitude sickness barfing. It's not super high, but the rate of ascent catches alot of people by surprise. Gold Bond your chafe points.