Detect touches on a surface
Arduino has a Capsense library. If your object is not metallic then it can be the perfect answer. Its cheap and easy to make too. You just need two arduino pins, few high value registers,aluminum foil and some capacitors.
This thing works on the principle that the aluminum foil works as a capacitor and combined with resistor it charges in time equal to the RC time constant. You send a signal to one of the pin and wait to get the signal of second pin. As a human being starts to put their hand close to it the time constant changes and you can detect that see if a human being is near by.
Here is a link to the arduino library.
EDIT Sorry I think I was hasty in posting. This is only applicable if you are detecting humans or anything that imitates human skin.