If you want to get a break from the Tokyo crowds and concrete, Tamagawadai Park (多摩川台公園) is a good place to spend your afternoon. Instead of walking to this park from the nearest station Tamagawa, I really recommend you to get out at Den-en-Chofu station. It takes about 15 minutes to get there, but Den-en-Chofu station is beautiful and definitely worth the extra walk. Around Den-en-Chofu station you can enjoy some traditional (and a few modern) shops and try out Japanese sweets and Matcha.
Tamagawa Park is famous for it’s Hydrangea flowers. It’s one of the most popular parks in the Tokyo area where these flowers grow. During the Sakura period, you can enjoy the many Sakura Trees planted all around the park. You can find other plants, trees and flowers as well. You can easily spend an hour or 2 in this park. There are many (clean) toilets as well and although you can always bring your leisure sheet, there are many places to sit down for a break. Tamagawadai Park has a very nice, relaxed atmosphere and you’ll see many dragon flies and interesting bugs near the big pound in the middle of the park. There’s a special playground for kids as well. Admission free and opened daily.
Official website (in Japanese): http://www.nikotama-kun.jp/shop/fuukei_park_tamagawadai/