Dresses have become my summertime 'uniform', since they're a one-&-done solution to getting dressed when it's hot, sticky & humid outside!
I've attached this find in yellow because that's the version I have (posted a WIW about it) but the J Crew website actually has a white one too. My yellow dress & the white one are both 100% cotton, so they're lightweight (but fully lined) & machine washable too. You don't have to go for a maxi dress, though
I also recently bought a few different colours of the J Crew Painter boatneck top (they also have it in white) & they wash well, if you're looking for a cotton alternative to the linen top that Suz linked to above. Good luck!