The question is how bored are you? Check this out: http://www.thecardboardboatbook.com/
http://jflis.com/leisure/canoeing/cardboard.htm
And of course I liked this one a lot even though it's not made of cardboard - http://www.instructables.com/id/Build-a-Greenland-Kayak/
Don't forget your cardboard life jacket.
Inside the Viewfinder - My photography blog