i'd say these would all be good tunes for a nice summer day.
not all indie, but im assuming you're just looking for non-hiphop stuff...
not sure if you're into this type of shit, but certain kinds of house music go pretty well with summer, imo.
ie. like this tune which has some slick latin influences in it:
or something kind of mellow and chill like this:
percussions and synths on this are pretty summer like, not sure how else to describe it.
best for last: