This is a project I finished awhile ago while learning how to use the HT68F001 mcu from Holtek. It has a single button to turn it on and check that its timing, and an led for some feedback. Its powered by a CR2032 and with the HT68F001 mcu pulling something like 8uA while running, should last for several years.
The idea is you have a drawer full of these and when you put your leftovers in the fridge throw one of the devices on it and click the button to start it. To make sure its running you can click the button again and it will blink twice to show its actually doing something. Once the week is up, it blinks continuously, letting you know its time to throw the nasty leftovers away.