Last year we made a different thankful tree (see here). It was more of a calendar and I modeled it after one I saw in the Pottery Barn catalog. This year we started a "thankful tree" with a basket full of blank colored leaves next to it. Every day my daughter has filled one leaf out with a thankful thought she has.
This is a really simplistic project that the whole family can do.
I really like the white tree with the colorful leaves and living in Hawaii during Autumn requires a lot of indoor fall decor, so that we remember that it's not summer. I'm also thinking that maybe for December we can do a scripture tree using the same base.