Make sure you do something good and geeky to celebrate. Like wishing everyone in your meetings a Happy Pi Day (which I can assure you will offer you nothing but a multitude of confused looks) or better yet a Happy Einstein's Birthday (which I can assure you will offer you nothing but a multitude of rolled eyes).
Or maybe you can kill a few trees and print out this MILLION DIGITS OF PI. MILLION. That's, like, a lot of Pi. (heh heh)
Mmm, speaking of a lot of Pi, you could always bake a Pi pie and share it with your coworkers. #nomnomnom
And don't forget to include some activities for the kiddies.
Any other ideas? Leave a comment!