Actually, there are four words that the Pnyx wish fountain will respond to... None are actually useful, and they are immediately overwritten by the default "Your wish is found elsewhere" answer, so they're almost impossible to read.

Pony - "Oh, that's original..."
Three - "You want three wishes? I don't think so..."
Coffee - "Seek ye Caribou" (author's favorite brand, I assume)
Tequila - "Galardon - 100% blue agave Anejo"

I stumbled across "Three" by accident (trying out words from Magpie's riddle about land and sea). Given one known word, I was able to dig out the complete list with MacsBug.