My daily "uniform" in the summer heat has been BF shorts and various tops with various sandals. I try to keep them from being frumpy by wearing a little jewelry or adding a belt, etc. I've really been wearing woven button downs more often this summer and Angie is right about them being cooler much of the time than knits. But in #1 that vintage cotton tee from jcrew is very cool and slouchy and I do have several of those.

BF Shorts: KUT from the Kloth
Black T: jcrew vintage cotton
Floral top: jcrew "Liberty" print from a few years back
Sandals: Teva
Belt: Gap