Funny, my 12 year old girl leaves small plastic ninjas in every cache we find where a ninja will fit (even if there is nothing she wants to trade for. She likes taking the caribeaners (she has quite a collection) and marbles.
Our 6 year old girl loves any small plastic animals the most (fish, dogs, bugs, anything).
I tend to leave useful items-mini first aid kits, hand sanitizer, etc. I love patches, so I think my favorite taken item was a geocaching patch. Maybe I will have some patches made as a sig item...