Here is an excellent article about one way to use some summer time to get ready for the coming school year.
Of course, it would help to officially know what I'll be teaching next year. I like to start summer planning by rewriting curriculum maps for my classes and reflecting on how the order of the curriculum worked last year. And of course, watching movies from Spanish-speaking countries that I might be able to incorporate into my classes next year. But on a micro-level, I have a hard time planning individual lessons without knowing which students will be in which classes and if they'll remember anything by September.