Ask the preschooler grandchildren what they really want to do, and if safe, do it. Riding a horse ... research local stables and petting zoos open. If they want to go to a theme park, look for small local carnivals to attend. Go see the animals can equal donating pet food to the pound and seeing the critters there.
Ask them what they want to be and then do an impromptu career day. For example, go take cookies to the firemen and let the kid talk to them about what it is like to be a fireman.
Talk about being slightly out-of-sync...!
However, thank you for bringing in some serious suggestions to what started out as a joke thread.
Any time you can get your grandchildren outdoors, away from the electronics, is a good day. Fish pond, rifle range, archery range, are all good. My son took his daughter out one winter morning to sit at the top of a canyon, in the snow, with a thermos of hot chocolate to watch the sun rise and see what critters would show up. She saw deer, turkey, and javelina, and remembers that day fondly.