Connect an LED to the output of an audio source in such a way that the LED flashes with the source. Then attach an applified speaker to a solar cell and presto you are communicating via light.
<http://www.exploratorium.edu/square_wheels/modulated_led.pdf>
The Exploratorium has a series of books called "Snacks" which explain how to make cool science experiments. Unlike some other books, though, the experiments actually work, and also unlike other some other books their books come with correct explanations of the science behind them. Check out a selection at
http://www.exploratorium.edu/snacks/
You can find the books at most big public libraries. See a list at Amazon here.
Oh, full disclosure, I work/volunteer with the Exploratorium, but I've never worked on a Snack book.