Children are natural collectors. Sorting and categorizing a favorite collection is a great learning activity and a lot of fun. (If your child doesn't already have a collection, encourage him or her to gather items while you are out and about in the world. Shells and rocks are durable collectables and can be rearranged and sorted in many different ways — by size, shape, color, and texture, just to name a few. If found collectables aren't an option, dried beans and seeds from the grocery store, building blocks, and other small toys work well too.)
Together, observe each item in the collection closely and encourage your child to describe its color, shape, pattern, and texture. Try categorizing by one characteristic; putting all the smooth shells together, regardless of color or size, or all the blue shells together, regardless of size or texture. You can sort small things in empty egg cartons or muffin tins. Bigger things can be sorted in larger containers, like shoeboxes. As an extension activity, you and your child can create a permanent display of the collection, making labels to describe the categories.