Usually there are small trinkets like little toys or erasers or stickers or magnets. Some caches are more "kid-friendly" so they are larger and have more toys; some are micro so they just have a log. What I do is bring out a bag full of things just floating around in my drawers that I'm never going to use (dinosaur eggs, bouncy balls, old keys, stuff like that).
Some people make little cards with their name. There's a cacher in town called The Hipster who does this, and someone who makes their own Pokemon cards.