I think your idea sounds great, but I'm just wondering, is your purpose letter sounds or sharing or just-for-fun? The reason I ask is because kids love to bring things in, but if you are truly trying to teach them to listen deeply and respond well, then usually talk shares work well because there is nothing in their hands, they are not thinking about what they want to share, and they really share important stuff and learn to talk back.
Maybe consider topics (like a poster above said) with themes (a time you were scared, sad, excited; tell about a cool relative, etc.) Listeners must learn to get outside themselves and respond in a way that addresses the sharer. When the sharer is done sharing, he/she can ask for questions or comments. Comments can't be about the others, but about the sharer: That really must have made you feel ____" Questions need to be genuine, "Where does your relative live?"
Not that show-and-tell isn't great too, but I've tried it, and I've found that the things they bring in get either more and more inconsequential or more and more grand (no matter what the topic/theme is!). I hate to unwittingly shine a spotlight on the gap between kids who have and those who don't. I really like pumpkinpatch's version of show-and-tell because even though they are bringing something in, the restrictions are tight enough that it evens the playing field.
Have fun whatever you do!