Even among Tokyo's innumerable flower-viewing spots, Ueno Park stands out with its sheer scale and tasteful lantern decorations. Turned on at sunset, these beautiful lights help make Ueno's nighttime sakura a must-see spectacle in itself.
During the day however, you'll find stage performances at the Takenodai (or 'Fountain') Square. These include dance, kimono shows, pop idol performances, an unusual orchestra where the public acts as the conductor (Sunday March 24), a Big Japan Pro Wrestling match (Wednesday March 27, 12.30pm-3pm), and many more. Check out the entertainment schedule here (in Japanese only).
Note that the festival period may change depending on the cherry blossom blooming dates.