i haven't done anything like that, but i LOVE the idea of God's backyard. that is awesome!
my thought for a tree would be to use maybe paper bags, crumple and fill with newpaper, and keep adding to each other and stacking it up along a wall. then it could either be covered with a fabric (brulap maybe) and then painted, or just painted. you'd have to play and create a bit in order to make it work, but it could.
you could also doa plaster tree.
i really like the idea. i hope you'll post pictures when you finish the decorating!
kate