Declutter organize repurpose has several really creative ideas for how to take a cardboard box and make it into a toy including a gas pump, a car, a refrigerator, doll bed, castle and even a shape sorter for toddlers.
Get the instructions here:
http://declutterorganizerepurpose.wordpress.com/2011/02/28/repurpose-cardboard-boxes-into-kid-craftstoys/
Five little chefs shares a tutorial for how to make cardboard box cars too.
Get the instructions here:
http://www.fivelittlechefs.com/2013/05/cardboard-box-cars.html
Not just a housewife decorates her car exterior too.
Get the instructions here:
http://www.notjustahousewife.net/2012/07/cardboard-cars-for-a-homemade-drive-in-theater.html
Laugh paint create makes a wearable cardboard car by adding straps.
Get the instructions here:
http://laughpaintcreate.blogspot.com/2010/10/creative-cars.html
Mum bo jumbo has doily wheels.
Get the instructions here:
http://mum-bo-jumbo.com/cardboard-box-car/
The merry though makes a camper with duct tape.
Get the instructions here:
http://themerrythought.com/diy/diy-cardboard-camper-playhouse/