Wikipedia: Pi – This site offers a lot of information about PI, including:
Fundamentals, History, Modern quest for more digits, Uses and more. This site has information about Pi Day too.

WikiHow: How to Celebrate Pi Day – Offers 4 ways: Enjoy Pi-Themed Foods, Pi-Themed Ambiance, Throw a Pi-Themed Celebration, and Celebrate the Real Meaning of Pi Day.

Time & Date: Fun Holiday – Pi Day – This site offers information about Pi Day and other Pi related days.

Teach Pi: This site is dedicated to Pi Day and offers videos, book suggestions, news, and more.

Exploratorium: Pi Day: One of the 5 activities offered on this page, Cutting Pi, requires a circular object, string, scissors, and tape. The other activities listed require easy to find items as well.

Fran’s World of Discovery: The Circle – The circle is a fascinating and versatile shape. The activity in this post encourages fraction use.

Scientific American: Let’s Use Tau – It’s Easier Then Pi – Article