My family and I just began geo-caching a couple of months ago, and we love it! (Wish we had more time to do it.) Beacuse we also have children, and we tend to go as a family, I really appreciate seeing kid-friendly items in the caches. In addition to some of the great suggestions already given, we often will leave a lapel pin from a location that we have travelled to. Both the young and young-at-heart seem to like these, and many people collect them, as well.
Happy geo-caching!