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.