This song is like black people's theme song. They play this at every summer party I've been to.
Follow Me - Aly Us

For a more obvious, less imaginative song choice there's Summertime. It's an okayish song. P. Diddy ruins it.
Summertime - Beyonce

This has always been the ultimate summer song for me. It came out during the summer and I played it non-stop the entire season.
Frontin' - Pharrell

This song just sounds like summer.
Hey Soul Sister - Train

And the one I have been playing over and over during the summer since '08 thanks to Dan Dan.
Nothing Can Pull Us Apart - Pilgrims on the Mind