The Air Wick Freshmatic Compact i-Motion air freshener is an intriguing target for re-purposing. It uses a passive infrared (PIR) sensor to detect motion in a room and then increases the rate that it dispenses air freshener. This air freshener sells for $8, but you can sometimes get it cheaper with coupons.
Radio Shack carries the Parallax PIR module for $10. So the question is why bother re-purposing the Air Wick air freshener?
If you just want to replicate the Parallax PIR module, you can modify the air freshener by drilling 2 holes, adding a wire, and cutting a trace. Stopping here, you end up with a can of air freshener, 3 batteries, 2 slide and 1 push button switches, a cool aerosol valve, a medium power PNP transistor, a two wire connector with leads, and a LED. I will show you two ways to do this; the simplest way, and a way to make the module the smallest.
However, If you want to go a bit farther, you can access the amplified analog sensor output as well as the digital signal, and have an externally accessible relay driver, and LED. I describe how all of these features are brought out to an easy to interface connector. You still end up with the air freshener, batteries, switches, and cool aerosol valve.
This last approach is a little more complicated.