One Box, Anything
A big box, some tape, and no instructions whatsoever.
Every blanket in the house, one afternoon, one secret headquarters.
A fort is not really an activity. It is a place where three more activities happen afterwards.
Building an enclosed space gives children something they rarely have, which is somewhere that belongs to them and that adults cannot easily enter. The engineering problem of making a roof stay up is real, and they will solve it themselves if you let them.
Nothing heavy overhead, and keep it clear of heaters, lamps and candles.
Agree the takedown time before you start building.
A big box, some tape, and no instructions whatsoever.
They design the board, write the rules, and then make you play it.
A cardboard frame, a white sheet, a lamp, and a proper show.
Teach them a game adults actually play.
Paper chains, lanterns and bunting made from what is in the recycling.
Find a smooth stone, paint it, leave it somewhere for a stranger.