The Tokyo Outside Festival has been an annual event for several years now, and is a true celebration of the great outdoors. The festival showcases different physical activities you can get involved with, a range of outdoor-based retail products (many at substantial discounts), and plenty of food stalls. Given the festival's timing, it's also the perfect opportunity to appreciate the beauty of the Japanese spring.
Entrance to the event is free, but do bring some yen if you plan on grabbing a bite to eat or drink at the food stalls.