The Lion, the Witch, and the Wardrobe
The first thing Lucy sees in the snowy woods of Narnia is a lamppost. (If you're wondering how the lamppost got there, well, you'll have to read The Magician's Nephew…we know it's kind of irritating that we keep mentioning it, but it is the prequel, so it answers all sorts of questions.) In The Lion, the Witch, and the Wardrobe, the lamppost functions as a beacon, showing the children the way to Narnia when the come into it from our world, and showing them the way back when they need to leave.
The lamppost is also a halfway-thing, not quite a real Narnian item (since Narnia seems a bit more pseudo-medieval than that – fighting with swords doesn't quite jive with having lampposts), but not quite something out of the Pevensies' England, either (it's too old-fashioned – it actually has a flame burning in it, not an electric bulb, which means it is from several decades earlier than the 1940s). At the end of the adventure, it is the lamppost which leads the way home for the children, triggering their memories of their old lives in England.
If all these things didn't make it important enough, the lamppost also causes Edmund to slip up and prove to Peter and Susan that he lied about his first trip to Narnia – the fact that he knows which way to go to find it proves he has been there before.