Establish An Exit Plan
The best way to avoid miscommunication about what things are (ultra casual) and when they're going to end (when the summer winds down) is to be straightforward about what you seek. Contrary to what women's magazines might lead you to believe, not every woman is in rabid pursuit of a commitment. Females dig flings, too. What we aren't down with is being led on.
When you embark on your summer romp, call that spade from the start. Tell her you "just want to have fun" and you may just be surprised by her similarly Cyndi Lauper-esque motivations.