Kids love looking for stuff and finding it! Give the kids a 'log book' - it can be just a folded sheet of paper with a list of nature items to spot and tick off.
If the weather is good to you and you can take the kids outdoors, either in your back yard, on a walk or even to a local park or forest center, then that's ideal. If you're indoors then you can still do this activity by printing pictures of the items.
Ideas: worm, leaf (for older kids pick a few different local varieties), beetle, y-shaped stick, slug, moss, ladybug, acorn, animal footprint, rabbit hole, etc.
You can take some small prizes to give out either to the child that spots an item first, or to all the kids when all the items have been found.