Mt Shirouma is the mountain after which Hakuba is named and is the most popular overnight hike. This peak can be tackled as a return trip from either the Tsugaike side or via Daisekkei (the snowy gorge), or better still as a round trip combining both legs. Either way it is a 6-7 hour hike to the top and will require you to spend a night either at the mountain lodge 30 minutes below the summit on the Daisekkei side, or at the adjacent camping ground. Many people get up early (before 5 am) to make the short hike to the summit in time for sunrise where you are often treated with a spectacular view of the valley shrouded in cloud with just the peaks jutting out. It is probably best to start from the Daisekkei side as it is easier to climb snow then it is to descend it, and it is a shorter trek to the mountain lodge/campground. From the Daisekkei side there is about an hour hike through the forest till you get to the snowy gorge then it is a steep climb, some on snow and some not. Make sure your boots are waterproof for this hike. From the Tsugaike Nature Park side it is about a 7 hour hike to the summit of Mt Shirouma via some very varied scenery such as highland marshes and a large lake which is often still be frozen over even in July. The last 2 hours involve you walking the main North/South ridge which gives you great scenery all the way on a clear day.
You can do this overnight starting on either the Goryu side or the Happo side and stay at the mountain hut/camping ground of either one. The Karamatsu mountain lodge is open from the end of April till early November whereas the Goryu lodge is open from the same date but closes about the 17th October and is also closed for a month from around the 7th May till the 12th June. The trek involves basically going up the Happo One ridge till you hit the main North/South ridge, then going along that ridge till you hit the ridge coming back down to the Goryu ski area, or vice versa. Being completely on ridges or summits for the whole duration of the trek, in clear weather the views are endless. Since you can make use of the lifts/gondolas at both ski areas it means there is less of a climb than for example the Daisekkei trek. The summit of Goryu Dake is about an hour hike in the opposite direction from Karamatsu so there is an extra 2 hours you need to factor in if you want to get to it's peak.