It only takes one giant cardboard box to construct an epic playhouse; use a box cutter to carve out windows and a simple door, and let your kids go to town with markers and crayons to decorate. If your kids are older or you can't find a large enough box, join multiple boxes together to make a box fort that puts store-bought playhouses to shame.
This activity is perfect just before or after a move, when boxes are plentiful, but you can also swing by the grocery or package store to pick some up just for fort buidling. Stack smaller boxes like bricks to form walls, and unfold a larger box to create a cardboard roof. You can also unfold multiple boxes and tape them together to form a large cylinder – cut a hole for the door and throw a blanket overtop and you've got a cardboard box hideaway [source: Cassler]. Another option – unfold multiple boxes and tape them together with duct or packing tape to form walls, then let kids decorate to create castles, playhouses or even an entire town.