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
Packing tape. Or of course DUCT TAPE. Duct tape is TOPS, and everyone should have some DUCT TAPE all the time.
A sharp knife.
An imagination; If you don't have one of these don't worry - the child will have plenty to share.
Step 4: Step 3
Step 5: Step 4
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.