Take time to pause and reflect on the experiences that have made up your undergraduate career. A senior synthesis project can spring from either the recurring themes that haven't been addressed as well as you'd like elsewhere in the curriculum, or the unanswered questions that your courses never seem to dedicate time to.
Think about which faculty would be most interested to hear about the themes or questions you've identified. The right faculty mentor for a senior synthesis project may be in your major or elsewhere, but they will ideally have seen at least part of your development as a intellectual.
As you focus on a particular experience or effort that might serve as a capstone to your Stanford education, try to draw its connections to the courses, tracks, activities, and opportunities that you have already assembled. How does your new idea complement, build upon, or react to those recent experiences?