This is another one of those activities that just kind of happened on its own.
My kids were coloring on the kitchen floor, and, for whatever reason, my daughter decided to go into the pantry and take out the bundt pan. Then both of my girls started filling the hole in the center with crayons.
I realized this was good for developing their fine motor skills (and keeping them busy while I was otherwise engaged in the kitchen), so I encouraged it.
They would insert the crayons into the hole, and once it was full, I’d show them they could pick it up, move the crayons, and start again.
You could use just about any small or skinny object for this: pipe cleaners (maybe cut shorter), pom poms, small toys or balls. Heck, you could even pretend it’s a volcano, and use small dinosaur toys. Let your kids get creative and pick what they want to use. Or compile a bunch of small toys and let them experiment to discover what will fit.