What should I do with a bunch of 16-17 year olds to get them interested in computer science?
You can have them draw pictures using context-free grammar. context free art This also works for people who never programmed before and scales to experienced programmers. The basic language is easy enough to explain in maybe half an hour.
Learning something about geometry using Turtle graphics should be nice too. Logo was designed for children, so highschool students should have no problem. There are nice videos about children using Logo on youtube
If you can get your hands on some MindStorms robots, programming them is lots of fun.
There are a variety of programming games in which you program robots to fight each other, or assembly programs that try to overwrite each other in a virtual machine. Wikipedia on the topic, related stackoverflow question
You can also think about some kind of hardware project. Making a microcontroller blink a LED depending on the number of unread e-mails in your inbox for example.
Have them implement different maze generation algorithms, try to come up with criteria that make mazes "difficult for humans". If time permits extend the algorithms to include not only corridors but also rooms.
Buy a couple of Arduinos and LEDs. Let them program the blinkenlights.