Many employers will tell you that extracurricular activities are just as important as what you study at the U. This is defintiely true and the reason for this is that activities in organizations teach you valuable real-world hands-on knowledge that can be directly applied to your future work place.
Grades. Grades are important - your average grade is important, but you should also excel at something.
Fun. Fun is important too! You can't be a student wihtout living like a student too - or, at least, that would be a bad idea... Fun minimizes the risk of not finishing your studies!
So, how do you find the right balance, and how can you make sure you are performing according to the plan? You need to make priorities. If you are good at theoretical stuff, you should definitely put enough effort into the things you do at university to ensure good grades. Try to identify your main interests and put your main focus into that area - ensure a consistent record: A's in all math and computational exams will ensure you a good tech job!
If you aim for leadership - engage in off-campus activities. Find an organization that fits your interests and seek out your position in that organization.
For fun: find your hobbies, your friends and interests, but choose strategically. Your activities in this category should be refreshing your life and your mood and should not take all your time.
When you have mapped out a strategy - define key performance indicators in each category and track them (number of pub visits with Joe per week, pages read per day in core subjects, etc.). Write down the KPI's for each week and adjust your activities to ensure meaningful levels. Find meaningful levels of each KPI by giving it some though - and adjust as you see how they work. Once in a while you should evaluate if the KPI's are right for you, are you moving in the desired direction? Are they beneficial to your life? If not, seek out others, or even reconsider the entire strategy. But remember, give the system time to work before you evaluate!