Panda Bear is a good choice and Animal Collective are really cheering up too.

Other recommendations:
- The Sonics
- Gotan Project
- Whirlwind Heat (Do Rabbits Wonder lp)

The Perfect record for being on the road with opened windows and opened sunroof (if you have one) is the Dead Man soundtrack by Neil Young.
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happy summer music for me is ALWAYS

the breeders
the flaming lips
sonic youth - washing machine/murray street

I can't really place the reason why, by I love Aphex Twin for a backyard chillin' soundtrack.

CSS is a damn happy music.