Whether you are grading a piece of writing, a collage, or any other kind of creative assignment, assign a number from 1 to 5 for uniqueness. This is your appraisal of how different the student's work is from other projects completed for the class. Students may have a little trouble adjusting to the idea that they are supposed to be different, because they spend so much time trying to be correct in their school work. Tell them you will be grading them on how unique their thinking is.
Synthesis is the ability to put ideas together. Grade students on their ability to combine elements in surprising ways. Assign a 1 to 5 grade for this by assessing how many connections the student make in creating the work. Connections include contrasting elements, complementary elements, contradictory elements and hidden meanings that emerge when two ideas are considered at the same time.
Creativity is no excuse for sloppiness. Beginning creativity is characterized by not understanding boundaries. Grade your students on their ability to create while remaining within the boundaries of good craftsmanship. This can include neatness, development of ideas, and accuracy of representation.
Grade your students on creativity that is useful. Wild imaginings can be fun, but include an element in your rubric that assesses the practicality of ideas. This can be particularly useful when the project includes creative problem solving. Let your students know you are looking for ideas that have a message that people can use.