As you plan for senior year, you may wish to work on an independent project to solidify the academic knowledge and other educational experiences that you have acquired during your time at Stanford. A senior capstone experience could be a project situated within your major or honors program, completed in close consultation with a faculty mentor. Examples of this type of capstones include an honors thesis, senior papers or projects, arts performances or exhibitions. Or you may be drawn to projects that are more interdisciplinary in nature, or that involve community partnerships. These projects also deserve close consultation with your faculty and other mentors on campus who can help you integrate those experiences.
You can find support for capstone projects from your academic departments or other campus programs such as the Haas Center for Public Service and UAR Student Grants. UAR Student Grants offer funding for research, arts, and senior synthesis projects. The "senior synthesis" as a project category supports senior capstones that are neither 'research' nor 'arts' focused. It emphasizes a reflection on your academic experiences at Stanford, and invites project ideas that go beyond what is required in departmental course offerings, with strong faculty mentorship.
For some examples of research-oriented capstone projects, consult Stanford News. This feature article from 2015 and a slideshow produced in 2012 will give you a sense of the range of possibilities.