One rule makes a snow day far more interesting than it would otherwise be.
What you need
warm gloves
a spoon or trowel
food colouring in a spray bottle if you have it
How to do it
Agree the one rule: it cannot be a snowman.
Each person builds their own sculpture, or work as a team on one large one.
Use a spoon for detail work rather than only hands.
Add colour with a spray bottle at the end if you have it.
Photograph everything before it melts.
Most activity lists ignore the things that actually decide it
Removing the default option forces actual invention, and the constraint does more for creativity than any amount of encouragement. Snow also collapses constantly, which makes structural problem-solving unavoidable.
Adapting by age
3 to 4 yearsBuild one shape together and let them decorate it.
11 to 12 yearsAdd a size or theme requirement, or build something that must stand upright.
Keep it safe
Waterproof gloves and a firm time limit. Watch for numb fingers, and come in before anyone is genuinely cold.