Anyone who has been around little kids knows that the new toy often gets ignored in favour of the box it came in. With this in mind I am sharing with you my super-simple play house.
Actually you don't have to do anything to a box to make it fun, but it adds something if a child can help create something. So here it is.

Step 1: Ingredients

One cardboard box. As big as you can get.
Packing tape. Or of course DUCT TAPE. Duct tape is TOPS, and everyone should have some DUCT TAPE all the time.
A sharp knife.
A child.
An imagination; If you don't have one of these don't worry - the child will have plenty to share.

Step 2: Step 1

Tape two laps at each end to form the gable ends of your roof.

Step 3: Step 2

Cut the fold opposite to your roof ridge.

Step 4: Step 3

Pull the walls into position and tape the flaps to the gable ends to give the walls strength.

Step 5: Step 4

Stand the house the way it should be and let the child's imagination take over.

You might like to cut some windows, add some extensions and doors, or slap on some paint. One time I made an ice cream shop from a box. The kids loved that one.

This house became a number of things in the first few minutes, until finally it became a see-saw.