I totally borrowed a lot of the ideas in this thread! (BTW, ya'll rock!) Our leprechaun dyed the milk green, TP'd all the green plants in the house, left a trail of green candy (Andies mints in the shiny green foil wrapper), and turned the toilet water green.
I didn't know if DD would go for it, but she completely bought it. Last night, she told me, "This has been the funnest St. Paddy's Day ever!"