We made this one not too long after we moved, out of one of the MANY boxes we had laying around. I'm particularly proud of the fact that the steering wheel turns, so it's even more like driving a real car. To make that bit of magic happen, I cut out two circles from another piece of cardboard; a bigger one for the steering wheel, and a smaller one for a hood ornament. Then I poked holes in the center of each and used a long twist tie to connect the two through a hole in the box (I just used the hole already cut into the box for a handle, but you could cut a small hole out of the side of a box with no handles). Draw whatever details you want with a permanent marker, and tada! We pulled out our colored pencils and colored the car, but that is completely up to you and your kiddo.
This one was created after Erik got a set of dishes and play food from his Grandma for Christmas. I hope someday to make a more permanent version of it (with this as my inspiration) but for now this is all I've gotten around to. It's great because he can store his toys in the box when he's not playing with them, but he also has a stove to cook on. The box I used had stuff printed on the cardboard, so I drew my stove-top on a piece of craft paper from the dollar store and then used contact paper and packing tape to attach it to the bottom of the box. (It turns out contact paper doesn't like cardboard, so the packing tape was required to get it to stick on at all)
This creation is technically not made out of cardboard, but it could've been (and it looks like it is since it's made out of brown craft paper) so I'm throwing it in here. I needed Erik to keep himself occupied for a few minutes one day and I was feeling frustrated that I couldn't seem to get him interested in his toys. I started lamenting the fact that he doesn't have anything fun to go with his collection of toy cars, and wishing I had the supplies on hand to make one of these. Then I realized I had a brand new roll of craft paper from the dollar store, so I grabbed a marker and threw this together. I realized as I was drawing it that the paper was too flimsy to last for very long, so I pulled out my roll of contact paper (I'm seriously falling in love with that stuff!!!) and put a layer of that over the paper. I had to use packing tape around the edges to make them stay down (remember contact paper doesn't like cardboard type surfaces?) but several weeks later our DIY road mat is still alive and kicking.