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.
Everyone is different, so they are going to want something different for their senior photo. I have some nice trees near my studio that seniors like to pose by. I would have options available, but don't limit people.