Ryugashi Cavern is a cool-weather escape in summer and one of Hamamatsu's better outlying sights. Part of the limestone cave is open to visitors, with stalactites, stalagmites, and a small underground waterfall.
It sits north of the centre, so it is more useful if you have a car or are already exploring the Lake Hamana side of the city. Pair it with Kanzanji or the lake rather than treating it as a quick station-area stop.