I found some great sites for instruments measuring intrinsic motivation. It always helps to know you don't have to reinvent the wheel. All 3 of my classes this semester have been very practical. I have accumulated a great deal of relevant, useful resources.

I still would like to measure creativity, even though I know it is difficult to measure. If I control for intrinsic motivation, perhaps by giving a post test with a high level of test retest validity after 2 projects, and then qualitatively look at the levels of creative thinking evident in the 2 projects, this could be a mixed methods approach.

The research problem becomes: Does intrinsic motivation affect levels of creativity in the high school art classroom?
If the intrinsic motivation is the same then creativity levels should stay the same. If they vary, then intrinsic motivation does not  play a large role in affecting creativity levels.

Let's look at it another way:
Use intrinsic motivation, test for it, when assigning one project (or in one class doing the same project) and then an external motivator for a different project (or different class doing same project project as first class).
Assess levels of creativity.
Research problem could be:
Do extrinsic motivators affect the levels of creativity in the high school art classroom?

Few points to consider about validity:

• If 2 classes are used how could I assess that the beginning levels of creativity are similar amongst the students in each class?
• What type of extrinsic motivator could be used? Contest with prizes are motivators in the art classroom. Grades could be used I suppose. One project they know they will get a grade on, and one they are told they will not.
• If two different projects are used within the same class, how to determine that both will produce the same levels of creativity without factoring in the motivators?