Take some aluminium foil and shape it into a tree:
make some more trees and put them in a box:
mould a forest floor to fit the box, attach the trees, and cover the foil with paper mache:
paint the paper mache:
drape the trees with vines, and add a monkey:
paint a background inside the box:
mould a frog out of coloured paperclay or plasticine:
Lots of fun and a lot of hard work! In case you're wondering, yes, I did help Lizzy with the fiddly bits - foil moulding, paper macheing, painting trees - but she painted the box, made the animals and flowers (with a bit of guidance), cut out the fern leaves, and stuck on the labels by herself.
And if you're feeling at all inadequate, my fellow mothers, about all the rainforest dioramas in a box which you have never made with your children (yes, I know how you think) keep in mind that we have never made one before either (I've never even done paper mache with my kids before, I admit it) and will probably never make one again. I'm hoping Ben will choose a rock pool habitat for his 3D project when he's in grade 3. Cardboard starfish in a bowl...now that sounds easy.