Apples in Stereo are the most perfect band to listen to when it's sunny and warm outside...
Also, a lot of the beachy-sounding bands...some stuff by Beulah, Rooney, The Thrills etc.
Modest Mouse and Phantom Planet are usually nice during the summer for me, as well as Kings of Comvenience and Weezer. Air is the BEST band to listen to while you're lying at the beach. And, ELO is liquid sunshine...