Not sure if this has been posted elsewhere, but looked sort of fun. Much harder to do in the UK though.
http://www.cbc.ca/news/yourcommunity/2012/12/cyclist-turns-new-york-into-giant-gps-holiday-card.html
http://www.cbc.ca/news/yourcommunity/2012/12/cyclist-turns-new-york-into-giant-gps-holiday-card.html