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.