If you're in the mood to create something fun and outlandish, and with some adult supervision - then the following cannon by Zach Ratter may fit the bill. The cannon is formed from some PVC tubing with electronically-controlled sprinkler valves used to allow pressurised air flow and launch a projectile. For some relative safety and Arduino circuitry, the cannon is controlled remotely from an Android phone with bluetooth that communicates with an Arduino on the cannon to control the valves. The following video contains a quick demonstration:
Even if you're not making a cannon - the project is a good demonstration of Arduino, Android and external hardware interfacing. Remember to heed the creators' notes with respect to the use of PVC piping, and be careful with the final product. Visit the project page for complete instructions. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
Another way of interfacing Anrdoid phones with Arduino is with a boad that has a USB host shield, such as our Freetronics USBDroid board:
Apart from being a fully Arduino Uno-compatible, it has onboard microSD socket and the full USB socket and host-mode controller. As well as the Bluetooth application described above, this is the ideal platform for developing peripherals or projects based around Android devices with ADK (Android Developer Kit) functionality, but without requiring a USB host controller shield stacked onto an Arduino. For more information and to order, visit the product page here.