Easter Eggs.

Not sure about your game, if it's a deep campaign world or just a game of "kill the orc" but I like to litter "explorable areas" with mundane loot that fills in details about the world. A journal of a lost explorer. Some coins with symbols the party doesn't recognize. Relics of an ancient kingdom.

You'll need to know your player's tastes so that the items have some value to them, even if not monetary value.