Hi I'm a preschool teacher and work with children 18 months to 4 years. I am in a quest to start a loose parts collection to teach and inspire creativity. This could be natural or upcycled:
Important things to consider:
They need to be big enough not be a choking hazard.
Look for things that are sturdy
They need to be beautiful

Ideas:
Natural
Sticks, A tree branch cut into cylinders, rocks that are flat, rock on a specific color, driftwood, pine cones separated by size or shape.

Upcycle
wool spools, Collections of caps and lids, cans that are not sharp, nuts and bolts, old frames without the glass, wood ends from floors, CD corks, Keys, wood curtain rods, sets of small boxes…and so much more

I added pictures for inspirations

preschool loose parts materials (benicia)