I'd also add Hama Rikyu Imperial Garden, particularly if your visit happens to fall in late March/early April when the cherry blossoms are in bloom. It is nice year round but magical during sakura season. An added benefit is that you can get there from Asakusa by catching the water taxi, which will do round trips to Hama Rikyu and then out to the mouth of Tokyo Bay before returning, or one-ways. It works the same in reverse if you would want to visit Asakusa after seeing the gardens.