If you feel that you can improve your GMAT score, you should retake it. I'd also suggest that you gain more experience and spend the next year or two developing your leadership profile (inside and outside of work).
Onboarding new hires would not count as community service - might help with demonstrating commitment to your company, but not community.
Mentoring senior employees is good, but this seems like an informal, ad-hoc activity. To some degree, all junior employees guide and inform their managers' decisions (well, not all!), so I don't know how you would be able to position this story as a unique strength. Hard to make it into a concrete achievement or story. Sorry