I pinned this site because it has a huge list of creative activities to do with your 1-4 year olds. Many of them are sensory activities. Since I needed to keep Lucy entertained while I cooked, and I had a couple eggshells from baking brownies, I chose the eggshell sensory activity. Basically you just let them play with clean and dry eggshells. I only had two, but it would be great to save them up until you had a good sized bowl full. You could take them outside with a spatula and let them smash, stir and play.
At first, she just kept tossing them in the sink. I guess she knows that's where they are usually supposed to go. She finally got the hang of it and had a good time crushing them between her fingers.
I would encourage you to take a look at the list and see what appeals to you and your little ones. We'll be doing a lot more of these as it warms up and we can play outside for the messy activities.