Kids love growing crystals.
The crystal shown in the photo appears to have been grown on a colored yarn string. It is interesting that the yarn seems to wind its way through the crystal.
I am finding that the easiest way to start a science fair project is to decide on a topic, such as growing crystals. Then find an exploratory experiment, an experiment that gives instructions for growing a crystal. Next, identify all the variables in the experiment, and from these variables, pick one that you want to test.
For an exploratory experiment about growing alum crystals, see