Actually, it was only one person who found me by doing a google search on "activities for the green book by jill patton walsh," but just one person is enough to make me want to be Helpful.
The Green Book, for those who haven't read it, tells of a colonising expedition to a planet where life is silicon based. When all their crops start turning glassy, and their rabbits die from eating the grass, it's not clear at all whether the new colonists will make it....It's a great book, and here's my full review.
Each colonist gets to take only one book. Pick your book. Read it over and over again. Bonus points if you can find a copy of The Pony Club Rides Again.
Research what happens when people eat ground glass. Do not try feeding it to your rabbit.
Dig a garden patch in your back yard, preferably in New England to maximize stone moving fun. Stone moving can be repeated every spring.
Get blank book with a green cover. Write a story in it. Call your story "The Green Book." Daring children could experiment with other colors.
Apologies to the original poser of the question--if you come back, do let me know if you thought of any more, um, activity-esque activities!