Ayu has beautiful summer songs -
BLUE BIRD,
Fairyland,
July 1st,
Glitter,
Greatful Days...
Tohoshinki - Sky
Mihimaru GT - Punkish
Otsuka Ai - PEACH
AAA - Hurricane Riri
Yuna Ito - Mahalola
YUI - Summer Song
Tohoshinki - Summer Dream
NEWS - Summer Time
Namie Amuro - WoWa
BoA - Valenti
Arashi - Love So Sweet
BoA - Shine We Are!
Every Little Thing - ファンダメンタル・ラブ
I don't like them all, but they all kinda have that summer feel
I hope you'll like some song^^
EDIT: I remember more lol
AZU - CHERISH
AZU - Rainbow
Morning Musume - Love Machine
Morning Musume - Renai Revolution 21
Hitomi Shimatani - Falco