The wife and I are new to this also. However, after 44 caches found, I can tell you I see way too much geotrash. And, it looks like the trade equal or up is mostly ignored. We leave a small compass or button. (new, not old and nasty). I like the earlier idea of batteries. Most GPS units run on AA, how cool would it be for a cacher who forgot GPS food to find a cache with fresh nutrition for his/her unit!
Whatever you choose to leave, I would suggest it be new or in excellent condition, something another cacher may find fun.