Build Your Own Magnetometer
One of the neatest gadgets made possible by modern IC technology is the magnetometer. For less than $25, you can construct a fun and durable tool to measure magnetic fields (static or varying) produced by currents, permanent magnets, pieces of iron, or the Earth itself.
Channel Surfing With An Arduino
The Arduino is perfectly suited to emulate the IR remote controls associated with many modern appliances. If your TV recorder and tuner are different devices, this will allow you to change channels between scheduled recordings on whatever schedule suits you.