Are you inside on a rainy autumn day with nothing to do and an energetic child who loves helicopters (and planes and trains and buses)?
Here's a tip for an activity you can do with him (or why not her?), old school or maybe Montessori: build something like a paper helicopter using sticks from ice creams you ate during summer and cardboard paper? Really easy,; I found this in a kid's magazine.
All you need is cardboard paper, 4 wooden sticks like the ones for ice creams (the "propellers" are from Magnum), glue, a cardboard tip from the separators inside the boxes in which eggs come in - goes under the propellers. My 3 year old was happy to cut, fold and crease (as instructed by me) and even happier to play with his new toy. There is room inside for a little toy friend.
PS. I also had a baby screaming for attention so I made a mistake and put the windscreen on the side - the green paper is rotated 90° - but I actually like it better this way, otherwise I would have had to paint a white square in front.