Photo source here.
We collect our shells (and sometimes sand or rocks) and while we are away I buy a nice jar or vase or holder of some kind. When we get home we either label our jar like the one above or I write with a sharpie on the bottom where we were when we collected the shells and the date. Then we have something nice to display that is unique and very memorable. Many stories come to mind when you pick up a shell out of the jar or look at them through the glass. We have five jars/vases like this around our home from various places and I love being surrounded by them and their memories.
Sometimes you have to think outside the box when it comes to holiday souvenirs if you want to avoid the cliche magnets and tea towels.