A pie is a baked dish with pastry on the top and bottom (just on the bottom is a tart, just on the top is a gastro pub cheat). Easy as pie means very easy (so why can't those over-priced pubs get it right?). A pie chart is a representative circle divided into sections.
Pie in the sky* is unrealistic expectations or promises (possibly made when pie-eyed which means drunk) A pie can also be a piebald (black and white) animal or bird, a chaotic mess of printers' type or a former currency unit in India.
*Not to be confused with pie in Skye which is a picnic on a Scottish island.