Sittin' on the Dock of the Bay by Otis Redding

Wasting time, wasting time

Stoned Soul Picnic by 5th Dimension

What's not to love? Stoned. Soul. Picnic. If that's not summer...

Centerfield by John Fogerty

Baseball. It's played. On a diamond. In a park.

The rest of the list is at
http://bstar.wordpress.com/summer-songs/