Sensors!

People love to play with different sensors

Lady Ada’s sensor tutorial

An incredibly long list of sensors, illustrating the huge range of things that might be sensed

Wiki dedicated to sensors

Here is a short list of sensors we like:

Flexible Stretch Sensor changes resistance when stretched. The stretch sensor is a new way to measure stretch, displacement and force. The sensor is a flexible cylindrical cord .060-.070 in diameter, with spade or ring electrical terminals at each end.
$10-$20 depending on length (2″ to 14″) at Images SI Inc. or at Electronix Express. Images SI also sells the sensor “cord” by the inch with no terminals.

These sites each have a great selection of sensors, very reasonably priced, easily interface to Arduino, and easy for beginners to use:

Lady Ada and especially her money saving sensor assortment
Fun Gizmos