Hello,
I have been asked to mentor at a summer science camp and to come up with a couple small projects for complete beginners at Arduino. The kids that I will be mentoring are between ages 8 and 13. I want to have the kids modify some values in code and interface with some basic sensors to control some LED's. I also am looking for some more advanced projects for me to do that I can show off that would provide that wow factor to motivate them. Any ideas? Thanks
At the top of this page is a bar, click on Playground, select "Interfacing with hardware" on the left ..... wow
Using number on the screen to show that a gyro sensor is working, that is not interested.
Something with motors, gears, or trains. Anything that rotates or moves.
Perhaps an air powered flute or organ with solenoid valves.
Or a cube with many leds, but that requires a lot of hard work to make.
For the more technical kids, you could use a very sensitive absolute pressure sensor (barometer) and show that if you hold the sensor high, the air pressure is already lower.
For the children to do, how about a 'hand capacitance' LED dimmer. Once it's powered up, they can see it working. They could then go on to do something useful with it, maybe operating a solenoid to unlock a box when a hand is held over the sensor. As an incentive, place a chocolate bar inside the locked box, which can only be retrieved when the project is working successfully. ]
Any project involving manipulation of LED's and/or motors through sensors are good ones, they provide direct visual responses to user inputs. There are loads of ways to provide input: temperature, pressure, sound, light, etc. One very simple example is to use a phototransistor as a light detector and have it control the speed of a motor... wave a hand over the photodetector and the motor starts and/or changes speed as if by 'magic'... 
You could maybe have a number of such light detectors ( up to 6, one for each analog port on a bottom of the range Arduino ), and have each connected to a solenoid that activates bells of different notes. To make it really fun you need to make a game of it with a prize to be had. Maybe connect the sensors up to some RC servo motors and they have to manipulate the light to get all the servos to point to the prize... the possibilities are endless...
Here is a project I did with LED's. I was going to make an old school arcade game ( albeit very low resolution! ) with joystick input but never got round to it. I didn't realise how much work would be involved when I started, LOTS of soldering...! 
It's fairly advanced using a stack of shift registers, PWM and timers but shows what can be done with a bog standard low end Arduino....
An Ethernet shield would open up the possibility for the kids to use their web enabled cell phones for web based projects, sms messages and email notifications, and similar activites.
Yup, either sms to some sort of display, (large scrolling LED?) or control a robot chassis from a smart phone (via bluetooth) will have the "wow" factor.
http://store.curiousinventor.com/sms-to-scrolling-led/
https://play.google.com/store/apps/details?id=name.antonsmirnov.android.arduinocommander&hl=en
App Inventor
Using android mobile to control arduino - Exhibition / Gallery - Arduino Forum(
One that is really simple that has a lot of replay value is a Jeopardy or Knowledge Bowl type game controller system. You have two large push button switches, one for each team. Or more if you want multiple players on a team to be able to participate. You reset the system and it ticks the seconds by, indicating that it is armed and ready for action. Then you read a question or flash a picture or have some sort of event that needs the player to decide whether to push the button. The first one to push the button wins (or loses, depending) and lots of lights flash and phaser sounds fire or whatever. You can add a scoreboard for additional eye candy.
For Science camp, you could do Science questions. Which element is Fe? Is baking soda an acid or a base? Was Copernicus a Biologist or an Astronomer?
Or you could add a computer monitor, and have if flash pictures of animals. You push the button when it shows an insect. If you push the button on a spider you get docked.
You can't go wrong with these two for something kids can pick up and play with -
Duane B