Kids love playing in forts, so when a couple of big plywood packing crates were being disposed of from work, I snaffled them and with a bit of modification converted one into a castle with drawbridge for inside play, and another into a playhouse for the garden.
Step 1: Materials and tools
I used a cordless drill and a jigsaw for this build, plus layout tools (measuring tape, pencils, square).
Materials were principally two large plywood packing crates, plus I extracted some rough-sawn planks from a heavy-duty pallet. I had a few 2x4s and some 2x2 cedar leftover from a deck rebuild. I also bought a roll of building paper and enough cedar shakes to cover the roof for the playhouse. We used a carpet offcut and some paint and polyurethane for finishing, plus some fence hardware (hinges and latches). Overall cost for the two forts was about $50.
Step 2: Playhouse
I mitred together a rectangle of 2x4s whose internal dimensions were the same as the box (this gave the roof a slight overhang). Over this I framed 4 rafters with a 38° pitch, roofed it with ply from the top of the box, stapled building paper on then shingled it with cedar shakes. The door and windows used fence hardware (hinges and latches; the latch can be opened from inside with a drawstring). The windows fold down; fortunately, the roof overhangs enough that it tolerated the windows being left open a lot. The exterior got painted with polyurethane and the interior was painted white.
A porch, curtains, carpeting and furniture were later additions.