I would probably try to narrow it down by asking what sort of look they desire over all. Is there a theme beyond "senior pictures"? Some people use "props" when they mean "background," so that's something else to consider.
If it's for a graduation, it's good to prepare props representing a hobby. Students are usually a part of organizations, which depicts that it has also been a big part of their academic life. It will be memorable if they can have it in their senior pictures.