Young kids love to collect rocks, sand, and shells while on vacation. When you return home, put those special objects in a jar. (Bonus tip: always tuck a few plastic bags into your suitcase when on vacation; the bags are helpful in many ways, and give you a place to stash those rocks and shells!)
I kept a few jars of shells and rocks on my daughter’s bookshelf. Now a teenager, she had the idea of using those jars as bookends. They are mementos and functional.