Although the weather can get a little humid in the summer months of June, July and August, these are also the most festive months, with vibrant and interesting matsuri (festivals) held all over the country. From the famous Gion Matsuri in Kyoto which has seen floats and parades snake through the streets since 869 AD, to the Awa Odori dance festival on the lesser-visited island of Shikoku and lots of smaller village and neighbourhood matsuri which are unique to that area's particular shrine, you're never far from fun and festivities in Japan during summer.
Along a similar theme, summer is also the time for hanabi (fireworks). These impressive displays light up the skies across the country, and going to a hanabi taikai (fireworks festival) is an excellent immersion into Japanese culture. You'll see crowds wrapped into their yukata (cotton kimonos) feasting on various street eats such as yaki (various meats, fish and noodles cooked fresh on a hot frill or fried) or chilled noodle dishes (very refreshing on warm evenings).
Once you've had your fill of culture, it's time to escape the humidity of the city and head to the mountains for colder climes and climbs. The Japanese Alps which create a spine through Japan's largest island, Honshu, are a walker's paradise, with a mixture of trails to trek or hike, along rugged cliffs that overlook meadows vivid with Alpine flowers in full bloom. Alternatively, head to Hokkaido - an island in the far north of Japan - which is known for its skiing in winter and hiking in summer. This relatively untouched corner of Japan has truly incredible volcanic landscapes, UNESCO-protected nature reserves full of interesting flora and fauna, and numerous hot springs.