This page gives links to the recommended web sites:

The Site with the Largest Collection of Activities

The Nrich web site contains the best collection of mathematical activities for aspiring mathematicians
http://nrich.maths.org

Sites with Collections of Activities on a Specific Theme

Simon Singh's Black Chamber code breaking site
http://www.simonsingh.net/The_Black_Chamber/

A history of code breaking
http://www.secretcodebreaker.com/history.html