DIY Cardboard Guitar

Ikat bag has instructions for making a guitar out of cardboard, rubber bands and a paper tube.

Good for ages 8 and up.

Get the instructions here:
http://www.ikatbag.com/2012/07/cardboard-guitar.html

Minieco has a box version using rubber bands and split pins.  There aren't any instructions though, just visuals.  There are some helpful tips in the comments though. 

See the images here:
http://www.minieco.co.uk/cardboard-instruments/

Makedo has a more elaborate cardboard guitar and it's shiny.

Get the instructions here:
http://makedo.com.au/inspiration-insight/how-to-make-cardboard-guitar.html

The frakers acres take cereal boxes, popsicle sticks, yarn and wrapping paper tubes for an easier approach 

Get the instructions here:
http://thefrakersacres.blogspot.com/2011/03/happy-its-march.html

Zakka life uses a matchbox and popsicle stick for her version.

Get the instructions here:
http://zakkalife.blogspot.com/2011/06/kid-craft-matchbox-guitar.html

Lyndsxo glamour moms uses a small plastic container to make her cardboard version work.

Get the instructions here:
http://lyndsxo-glamourmoms.blogspot.com/2010/10/daily-play-cardboard-guitar.html

Martha Stewart paints hers orange.

Get the instructions here:
http://www.marthastewart.com/265694/making-music?xsc=eml_crd_2011_08_17&om_rid=NbR$fr&om_mid=_BOS651B8WNdJPT