This post may come across as arrogant since the tone is kind of know-it-all-ish. To counter that here goes my disclaimer: I may be wrong and what I've written is just what I would have said to my about-to-graduate self if he (I) had come to my 30-year-old-self for advice:
1) While planning a few things ahead would probably put you in a better position come application year, don't overdo it. Choose the employer and industry you feel you will enjoy the most and be passionate about your job. If you enjoy your job, then performing well will be a natural thing and you'll get the steady progress at work that will impress admissions people (specially since it's usually much easier to progress from entry-level positions than from higher positions). Don't choose your first job based on a token compensation difference, choose it based on how you think you would feel being a part of each organization.
2) Make sure you have a life outside work and try and make it meaningful to you. If that means joining Habitat for Humanity or United Way, then by all means do so. But if you wouldn't like spending your free time doing those kind of activities, then do something else instead. Just make sure you are consistent with the activities along time and that they are meaningful to you. If you do so, you will get:
a) more out of your activities.
b) better stories to tell and opportunities to lead.
3) Leadership is not about being formally in charge of a group of people. While managing people will provide challenges that you have never faced before and arguably the most difficult (and typically most important) challenges in your professional life, you can be a leader from any position in an organization. Leading is about changing the status quo (but only if it needs to be changed). Leading is not about coming up with ideas to improve things at work but about selling those ideas properly and making sure they become reality. Leading is about influencing other people to do what's best for the organization and it can take many forms: from keeping employees motivated to making sure that needed changes take place through just keeping things functioning properly if they don't need any fixing.
4) Update your resume regularly. Are you stalling? Analyze why and take action. Are you progressing enough or even investing a few years paying your dues to get better opportunities later on? Then stay put as long as you are content. But at the same time, have realistic expectations, don't just go job - hopping looking for the Holy Grail job. If it exists, it's usually very tough to get.
5) There's plenty of time to both have fun and keep a succesful career and life. There's no need to cut down on leisure to focus on resume-building extracurriculars. Actually (going back to point 2), there's no reason why these two (leisure and extras) should be completely separated.
6) The world is a huge place with lots of cultures worth knowing and places worth visiting. Try to get international exposure through work or trips - if possible - or by reading and talking to people.
7) Try to get a balanced view on all issues, know that absolutes are simplifications at best and wrong statements at worst. Be prepared to change your mind about things you thought strongly about as you mature.
8) Don't set your MBA plans in stone. Be honest to yourself and go for it only if it makes sense to you and your career / life vision.
Hope it helps. L.