I haven't come up with anything consistent for trade items, but I try to leave a little something and a token item. I made a bunch of sculpey clay disks and stamped each with a little hedgehog (I dunno why, I just like hedgehogs and had this stamp). I write the date and the number of the cache on the back when I find a log a new one. Other than that I've left marbles, pins, pens, small toys, etc.
EDIT: Having just read the welfare cache thread, I think I'll be upgrading my trades in the future.