Unfortunately in San Diego there aren't as many cool shells on the beach as you'd think. So as much as I'd like to take the credit for my mad skills in shell collecting, these are all store bought. But they are actually from my wedding in 2010, so I feel better finally finding a home for them!
Find/buy some great shells, some Elmer's glue, and anything else you want to get rid of (ha!) and let your kid go wild! The glue can get messy, but it's a good skill for them to learn how to control holding and squeezing at the same time (and in a line if you can get that far). And picking out matching shells, or shells of the same color or size is also a great lesson to teach while crafting!
To start, I drew the sand castle, or snail - whatever they want really. And then help them as they decorate their art! Even putting sand on the piece would be really cool after you're done gluing everything down and before it dries.