If you are looking for a project for a Bat Mitzvah around Shavuot, you could try floral arrangements with old nylons. Nylons can be dyed using store bought dye or food coloring and vinegar. There are a variety of examples on the web for inspiration and tutorials, or you can get a copy of The Art of Handmade Flowers.
For online tutorials and examples of nylon floral arrangements see:
Our own modest example with pipecleaners as seen in the post Tu Bishvat flower ideas.