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: