There is no "best" language. Developing an iPhone app would be fun and maybe impress some girls (or boys) :)
Learning Python/JavaScript and building a web app is probably more useful if all you have is one summer. It can take a while to get really good with Objective-C. That said, Objective-C is a superset of C, which is used in many computer science classes. If you're going that route, there is no better time to start learning than now!
Personally, I like the Big Nerd Ranch books. They might be carried by your local library. The Apress books are also good. Any of the beginner books are suitable learning tools; you just need the self-discipline to work through the entire book.
Learning Python/JavaScript and building a web app is probably more useful if all you have is one summer. It can take a while to get really good with Objective-C. That said, Objective-C is a superset of C, which is used in many computer science classes. If you're going that route, there is no better time to start learning than now!
Personally, I like the Big Nerd Ranch books. They might be carried by your local library. The Apress books are also good. Any of the beginner books are suitable learning tools; you just need the self-discipline to work through the entire book.