Make a Shadow Theatre
A cardboard frame, a white sheet, a lamp, and a proper show.
A big box, some tape, and no instructions whatsoever.
The most open-ended activity there is. Hand over a large box and genuinely say nothing else.
Open-ended materials produce longer, richer play than toys with a fixed purpose, and boxes are the clearest example. Resisting the urge to suggest a spaceship is the hard part for the adult, and it is the part that matters.
Adult does all the cutting. Remove staples from box corners before handing it over.
Flatten it for recycling only once they have genuinely finished with it.
A cardboard frame, a white sheet, a lamp, and a proper show.
Tape a huge sheet of paper to the floor and let them draw on their hands and knees.
Find a smooth stone, paint it, leave it somewhere for a stranger.
Every blanket in the house, one afternoon, one secret headquarters.
They design the board, write the rules, and then make you play it.
Two identical piles of bricks, a barrier, and one person giving instructions.