Take the boxes and open one side so you have a 'door' or entry point into your 'apartment building'. Have your child glue them together any way they wish. This makes them make creative decisions and tests their spatial awareness. They also will see what 'fits' together and balances, and what does not.
Once the boxes are secured, let the glue dry, and then have your child decorate the building any way they wish. Once complete, this apartment makes a nice home for tiny toys or a parking garage for dinky cars!