Character sketches. Find those quizzes teens LOVE that have random questions--fav ice cream, color, song, etc--and have teens come up with a fictional character to answer the questions.
Collages-- have them rip things out of magazines, bring in pictures, etc, taht represent elements of their story for inspiration.
Freewriting--write anything and everything they think of for ten minutes--it can be the start of their story, a dream the chracter is having, etc.
Have them learn to hone/write a hook. Give them examples ("Harry Potter" or "Twilight") and have them figure out how to write a concise, one sentence pitch for that book. it will help them learn to tell people how to asnwer that question "So, what's your book about?"