How do cognitive scientists measure creativity?
The Torrance Test of Creative Thinking is a good place to start.  It scores for fluency (number of responses) as well as originality (statistical rarity of responses) and elaboration (level of detail) on a variety of different tasks. The validity of the TTCT has been examined with several long-term studies, so there is a fair amount of data on its measurements as well as the form of the test itself.

Torrance, E. P. (1981b). Empirical validation of criterion-referenced indicators of creative ability through a longitudinal study. Creative Child and Adult Quarterly, 6, 136-140.