Some ideas for afternoon projects!

LEDs We have red, green and yellow LEDs -- you can make different patterns of lights, controlled by pushbuttons, control knobs or photocells.	Music Make music with piezo buzzers. Have the Arduino make up random music, or make tones according to user input from a control knob, joystick or light sensor.
Joysticks (4) A joystick is just two potentiometers, one in each direction. Use it to control lights, sound or whatever. Some joysticks include a pushbutton too.	Photocell (light sensor) Make music with a light theremin. Make an alarm that goes off when someone opens your backpack. Detect when a light turns on.
Tilt switch (6) Make a burglar alarm that goes off when someone tries to open a box lid. Make something that lights up when you shake it, or a pedometer that keeps track of how many steps you take.	Sonar rangefinder (4) Ultrasonic sonar rangefinder. Use like a motion detector, to tell if someone walks in front of your project. Or put on the front of a car to keep it from bumping into walls.
Stepper motor (4) Rotate things around. We have two types: one uses the Arduino Stepper library and the other uses the LadyAda motor shield. The LadyAda one is easier to wire up, but you have to download a separate library.	IR motion sensor (2) Make an alarm that sounds whenever someone walks past. Or make something that plays music or lights LEDs when you wave your hand in front of it.
Servo Motor (2) Move things back and forth, or around in a half-circle. Can be finicky; may need an extra battery.	Cars / DC motors (2-4) Make toy cars drive around. Maybe couple them with a light or IR motion sensors. Warning: motors are finicky. It's fun to get them working but they can be frustrating,