I got the idea from HERE and HERE.
We started with two oranges and a large clear bowl filled with water. I put both oranges in the bowl and the kids saw that they floated. They are floating because of the peel which represents the armor of God. We read several scriptures which talked about the armor of God and how we can protect ourselves from the bad influences of the world by putting on our own armor (such as telling the truth, being kind, saying our prayers...)
We also talked about things that might make our armor come off such as if we forgot to pray, didn't read the scriptures, or said unkind words. Those things would cause us to loose the protection of God's armor, and to demonstrate this I picked off some of the peel. With some peel missing the orange still floated, but as we peeled off more and more, it sank until it was touching the bottom.
I took this one step further and talked about how when we repent we can get that armor back on. I had to carefully unpeel another orange, making sure to leave the peel in one piece (since the first orange I had picked away at the peel and couldn't reuse it).
With the piece of peel surrounding the orange again, the orange floated. (I forgot to take a picture of that part, but trust me, it worked). The kids thought that was pretty neat.
This was a pretty simple object lesson to set up, demonstrate and clean up. Plus, it kept their attention the entire time. This was a win-win in my book.