As a high school senior, you have a lot on your plate(I know, I was there 6 months ago). Although the temptation to just coast into graduation is the most desirable option you have, I would argue that going full speed to (and through) graduation is potentially the best thing you can do for yourself.
Let's face it, even though you are finishing "school" in less than a year, if you are reading this post, then you are considering attending college. College is a completely different animal than high school, and the transition into your new life will flow much smoother if you don't slow down at the tail end of your grade school years. Also, colleges are beginning to look more at your extracurricular activities when handing out scholarships, and "Secretary of SGA" is a lot more appealing on a transcript than "professional sofa recliner."
Growing up, you've been told one of two things regarding grades: 1)Grades come first. or 2) Grades aren't everything. From my personal experience, both of these statements are true. As a high school senior, you should definitely devote time into study hours so that you can form a nice discipline of studying going into your collegiate years. However, don't make your studying and grades the only thing you care about in life. Get out, hang out with friends, play sports, do yoga, BE ACTIVE! Being a well rounded individual will put you light-years ahead of your fellow unmotivated classmates that are "just trying to graduate."
In short, don't coast into college. Study first, play second, and enjoy every minute of it. You can't get these years back, so cherish them. and the number one skill I would advise you to enhance is your communication skills. Be friendly, talk to people, join clubs, smile. You start to make life-long friendships in college, and the relationships you form with your professors has a real impact on who you can expect a decent recommendation letter from. Start honing your skills now, and like I said above, smile. Life is too short to not show someone your teeth.