Perusing a copy of Jack Heffron's The Writer's Idea Book, I came across a few exercises to share.
In one section, he discusses using form to generate ideas. As an example, Heffron tells about a M.F.A. poetry workshop assignment where he had to find a poem he liked, and then write his own poem using exactly the same meter. He said that by concentrating on getting the meter right, which was not an easy task, he lost his self-consciousness and came up with better poems than he thought he could come up with.
Here are a few of his prompts that are designed to help you get writing ideas by focusing on form rather than on subject:
Prompt: Find an essay or story or poem that you like. Outline it, noting turns in plot or shift in topic or approach. Write a piece of your own using the outline, simply changing the topic.
Prompt: Write a story based on a myth or a fairy tale, setting it in contemporary times. For example, you might retell the Hansel and Gretel story using two children you know. If this works for you, pick another myth and try again.
Prompt: Retell a myth or fairy tale, changing what happens or exploring character more deeply then the original. For a model, read John Gardner's Grendel.
Check out the book for more form-related prompts, and lots of other great writing activities.