The Motoara River Cherry Blossom Avenue is the biggest hidden gem. It's a bit out of the way from Tokyo, about 1-1.5 hours, but it's absolutely worth it.
Stretching along the river, this avenue is a stunning tunnel of pink during cherry blossom season and stretches on forever! There are many spots with several stones in the water where you can cross and sit. I went here on a Sunday and there was absolutely no one here. It is truly a cherry blossom gem 🌸