This craft popped up on one of the librarian listservs recently and it's so cute I had to share it: Seed Bombs. Basically you mix seeds and dirt to form a ball and then you throw it anywhere that needs some flowers. Find more detailed instructions here.

It also pairs perfectly with a very sweet book: Max's Magic Seeds by Géraldine Elschner.
Also try these fun books as you start to plant your gardens:
The Carrot Seed by Crockett Johnson
The Curious Garden by Peter Brown
The Empty Pot by Demi