Do's: Wear a fun-patterned 5-panel—floral, paisley, cats, hot dogs, whatever. It gives off the vibe that you can take it easy and go with the flow. I might be biased, but the Eagle 5-Panel from The Hundreds comes in my favorite plaid print of the year.
Don'ts: Steer clear of wearing black. Other than obvious scientific reasoning of "black absorbs the most light and therefore generates more heat," black is too serious. Summer is supposed to be fun and carefree!