Grow a Lima bean in your window. While this is not really a craft project, it will still be something fun for the kids to do. Once the bean starts growing the kids could always paint a nice pot to plant it in. Think I'll do this one with my cloverbuds.
This is a wonderful flower matching game. For preschoolers, this would need to be a small group activity instead of an individual activity so that reading help could be given to the child. This is a great way to re-enforce the parts of a plant.
Carnation colors-- this is a great activity for preschool to show students how flowers soak up water through their roots, how it goes up the stem, and out into the leaves and petals. This is an experiment that the kids could perform on their own and make observations each day about the changes they notice.