We've started keeping a little glass bottle full of sand from the different waterside places we've visited! We dry out the sand on a cookie sheet, but I don't wash it. It's neat to see the variance in the colors/texture of the sand from different beaches! If it's a craft you're looking for, I know that some people use the sand to make sand pictures or layer the sands in a larger jar.
Pressing flowers or leaves is another idea, depending on the season, I guess. You could use these in lots of ways -- making cards, sandwiching them between two pieces of glass, framing them.
Of course, we also look for an ornament for our Christmas tree that depicts the place some how! :
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My DKs ask traveling friends to send us postcards of places they visit, and we put them all in a post card album.