A lot of people make fantastic things with simple cardboard and I've always loved this material from being a boy making play houses out of it till my years at art-school where I made my final work out of it:
On Pinterest you get a lot of inspirational boards when you search for cardboard:
The Dutch artist Filip Jonker even made a tugboat with it and sailed it to Londen:
And of course you can find a lot of inspiration here at Instructables.com.
Handy program/app to use is off course 123D Make. You can print out you're designs and cut it out by hand
Now for the pics:
(I'll have to finish this one tomorrow because it is late enough, but I wanted it to publish for the cardboard and duct-tape contest)