Danish stand-up comedian Nikolaj Wulff ( shared this photo of his pet guinea pig getting REALLY excited about finding a piece of carrot. I have the same expression when I run across a slice of cake. Carrot cake, of course. Via I Can Has Cheezburger?
I Recently Sewed A Stack Of Guinea Pigdorable Peepee Pads. They've Been Super Handy! The Blasphemous Homemaker: Guinea Pig Cage Enhancements. This may work with other small animals. Like a small dog maybe.