5) Pieces of nature
If you want something truly unique, a little piece of the place’s landscape could be it. Within reason, of course — don’t deface an environment or take something you’re not allowed to. I picked up a couple of rocks near geysers in Iceland’s Golden Circle, and when I returned home, I covered them in a clear coating to protect them. In Scotland, I picked up some seashells from Cramond Island, and a little pine-cone from a forest in East Lothian. Remember to check customs regulations before picking up natural items, though, as some things — such as seeds — are forbidden.
Many people even collect little vials of dirt from the places they have visited. Literally, a little piece of the country they visited.