Survey individual student interests and then spend an afternoon finding a few appropriate activities for each. The student-driven learning that follows will make you look like a genius. There has never been a more direct demonstration of the way concepts cross traditional subject lines than the Internet. You're always a click or two away from looking at the same focus subject from a different angle. For secondary teachers, this is a way to put two or more classes together and enjoy a variation from the one-subject doldrums.
Every class has a few Internet greyhounds looking to race ahead.
If you don't involve them, they may violate acceptable use or delve into hacking. Give them lab responsibilities. Have them track web sites on the subjects of your choice. No teacher is an expert on everything. The world is full of individuals who would donate their expertise, but can't break away from their jobs to come to your classroom.