My grandsons range from 6 to 1 year old. Best thing is to get outside - boys are easily delighted with a ball, a stick, a puddle… a hill to clamber up or slide or roll down. Bike or scooter, take a drink and snacks, keep on the move. I have apps on my phone to identify plants and insects, the eldest has been fascinated with these since he was about 4. I pick up bargains at charity shops so there are always clothes at my house that can get wet, muddy or spoiled without parental concern. Swimming things live here too, and we go to the local pool through the holidays for a 90 minute fun session. Frisbee on the recreation ground, throw balls into the stream for the dog to retrieve, visit ALL the local playgrounds to rank slides and zip wires in order of preference. Take kites up to the field if it’s windy, or bubble mix. We get outside in all weathers, they have wellies, raincoats and gloves here. And afterwards we can settle on the sofa with books or a film, or do drawing, or make paper planes.
I much prefer having them here than looking after them in their own homes. Even if they’re here once or twice a week, the slightly less familiar setting makes it much easier to occupy them!