I did a quick writing activity with my 7th graders, and it was based on the book "All I Really Need to Know I Learned in Kindergarten" by Robert Fulghum. You remember the lessons - sharing, saying thank you and please, taking naps, enjoying a simple snack of milk and cookies.... Everyone can relate to its simplicity. As we get older, we can really appreciate those things in life. For teens, not so clear to them yet.
I read the first 3 pages of the book to them. It's a perfect introduction to those simple things we appreciate, and perfectly illustrates the task you are going to charge them with: show how a common object can teach us many life lessons.
Here are some the class has come up with. I showed them a sample of the finished product using my dog. I always model the expected outcome in all of my lessons - even the essays. I believe it is important at this age to set expectations for them. The others are the posters they came up with. I picked out a few to give you ideas.
I suggest you try it with your own classes. Mine enjoyed the challenge!