We recently got some new stuff which came in big but fairly flat cardboard boxes. I was about to recycle all the cardboard when I thought why not let the kids play with it first.
They had fun coloring on it:

We made ramps for cars:

I wrote the alphabet and played games- rolling cars on it and naming the letter it landed on or tossing stuffed animals and naming the letter. They also like tracing the letters with crayons.