I don’t usually recommend Ueno as a tourist destination (the zoo is just cruel, the museums are stodgy, Ameyoko-cho is charmless, and the park is ugly), but at the end of July and early-August when the lotuses are in bloom, the Shinobazu Pond in the lower section of the park is actually quite nice.
The lotuses will probably be in full-bloom after the 20th or so, and it’s best to go around 10 o’clock in the morning when they’re fully open.