I've got this nice cat pencil case as a present from a friend, who's sewn it herself.

To make it even better I've decided to upgrade it. I thought that it would be cool if it's eyes would light up green when it's mouth was opened.

That's how it looks on the inside.

A paper clip serves as a battery holder. Note that half of the paper clip is in heat shrink as otherwise it would just short the battery. Two green leds are soldered in parallel and sewn behind the eyes. They were regular 5mm leds, from which I've sawed off as much as possible with a dremel. When the mouth is opened the latch completes the circuit and the leds light up.