Must not miss to see hydrangea in rainy season.
There are several spots to see really beautiful hydrangea.

Maizurujo Park (Best period : Middle ~ end of June)
Northern part of the park is perfect spot to see hydrangea. As you relax on the bench.
Address : Marunouchi, Kofu city (3 minutes by foot from JR Kofu station)
https://www.city.kofu.yamanashi.jp/welcome/rekishi/kofujyou.html

Kai Zenkoji Temple (Best period : Middle ~ end of June)
Blooming hydrangea in the temple make wonderful mood.
Address : 3-36-1 Zenkoji, Kofu city (7 minutes by foot from JR Minobu Line Zenkoji Temple / 15minutes from JR Kofu station by bus, then walk 5 minutes from Zenkoji bus stop)
http://www.kai-zenkoji.or.jp/

Takeda no mori(Best period : end of June)
The forest locate high land bit away from center part of Kofu downtown.
So you can enjoy 1,200 hydrangea blooming later than downtown.
Address : 3371 Katayama, Yamamiya-cho, Kofu city (20minutes from JR Kofu station by coach)
http://y-zouen.jp/takeda/