Don't look for a club or activity just to pad your resume. Join a club that does something you like. Do athletics. Get leadership positions in those areas, and do well at it. Be a team player. That is what will get the attention of Admissions. Particular scores on tests are not a limiting factor.
Don't forget community service and having longevity in your summer/part-time school-year jobs. Demonstrate that you care about others and that you are responsible in more than just an academic setting. Seek not just participation but leadership roles in both of these areas. You can then provide the academy with additional strong letters of recommendation from supervisors who can speak to your character as well as your ability to work hard and be resourceful, qualities highly valued in the Coast Guard.
Everything is important. Anything that shows a wide range of interests and activities helps a candidate stand out.Is an Amateur Radio license taken into account or is it not important?