Here's a place to find not only lots of green craft ideas, but also craft supplies which you can make yourself from natural materials. There is a category for garden-oriented crafts.
What about giving little kits to make some of the craft projects? Using mostly recycle materials, this could be done at very little expense.
There are also a few projects that would even be appropriate as gifts!

I found this page VIA Green Crafts for KIDS on Pinterest. There are even more good ideas there.
                                                                                                                       Image from Planetpals