This is Kiyosumi Park (清澄公園, kiyosumikouen). If you wanna do hanami as a family activity, this is the place.
Tourist Info
Kiyosumi Park is a small park, equipped with artificial river, artificial pond, some playground-field, and athletic facility. There are several sakura trees in the park. There are not many sakura trees, so naturally there not many people in the park in sakura season; the fact that you can enjoy sakura without not too many people can be attractive to some.
Next to this park is "Kiyosumi-Teien," which is famous large park.
Access
0. From Tokyo station.
1. use Marunouchi line (metro) to Ootemachi Station. (1 minute)
2. use Tozai line (metro) to Monzen-nakamachi Station. (6 minites)
3. walk north (15 minutes)