The trick is to leave it open enough so that there is the possibility of surprise. This is not that difficult, because my students often do things I do not expect nor dream.
I teach three different studio courses: emphasis on creativity, story telling, character and metaphor. I find the trick is to not to let them do anything they want, but rather give them definite constraints – then they fling themselves against the limits because then the students know the limits’ location and also it is their nature to test them.
The constraints are objective: size, scope and necessary inclusions, for example: create a 30 second animation that incorporates a minimally bipedal walk cycle, an additional loop, and has a narrative arc – no death allowed. I include the last bit because it’s too easy to make a fight-the-bad-guy-end-in-epic-blood scenario. You know they test that. “What if he melts? or implodes? or is grievously injured?” “Does the character have to walk necessarily? Could they roll? What about a peg leg?”
That said. They consistently produce work that makes me happy. Not all: but many. Some will do the bare minimum and do a literal interpretation of the constraints put forth. Still, some have the capacity to make me go home at night pleased that I have this job.
Exhibit B: The shirt woot derby. Every week, the folks at woot have a “derby” where people submit Tee-shirt designs based on a specific them with specific constraints. The community votes on their preferred designs. Discussion in the comments ensues. The best (as judged by a combination of woot staff and community) rise, and the worst are rejected.