May I suggest an insect or spider? I know this seems like a disaster waiting to happen, but a colorful spider or mantis that is too big for the plant might work out.
Insect fecal matter, usually quite dry (they can't waste water!), is easily removed from the environment, unlike other animals' waste. Some bugs tend not to defecate until they eat, which means you can control it. Carnivorous insects or spiders do not require much care, and eat the same bugs your plants do.
Some spiders are gorgeous, and will set up shop in a terrarium quite happily. I had a mantis once that was nearly 6 inches long, and its single-minded killing/eating behavior kept me fascinated/horrified for hours!
--Steve
(Edited by steveo at 1:40 am on April 3, 2002)