So simple but fun, quality time with 4 yo:
1. A little walk in the forest collecting fresh leaves
2. At home put the leaves under a paper
3. Colour with crayons over the leaves.
I think these could make pretty cool cards too when cut and glued on a nice background.
We ended up talking about the leaves and trees so much that my daughter seems to recognise the most common leaves now. Not a bad unintentional outcome for just spending relaxing time together!
We decided to dry and press the leaves for future crafts. But since you already have collected green leaves, you may want to do some printing with them instead.