The biggest mistake you are going to make as an undergraduate is doing too much. You will take too many classes, choose too many majors, join too many clubs, get to know too many acquaintances, and generally treat your education as an all-you-can-eat buffet rather than as an elegant dining experience. How can you fix this?

Do not treat your undergraduate education as an all-you-can-eat buffet
First, understand the point of an undergraduate education. Simply put, it is to get to know yourself and in so doing to acquire the skills and knowledge towards a meaningful life, one that benefits yourself because you have found a way to contribute to others. When I say get to know yourself, I mean learn to reject the things that are not going to work towards your goals. Now is the time to learn you excel by specializing. Combine that specialization with passion and you will thrive. It can’t happen if you try to do everything.
But what if you want to try everything? What if you don’t know what you are interested in and need to try a lot before knowing? That can be fine maybe for the first year or two. Go to lots of clubs. Join different groups that are striving to make a difference to peers by tutoring or to the community in many ways. Take a scattering of courses to see what you like. Why not?
And why not keep going with too many activities, too many majors, too many credits per semester, and too little sleep? Isn’t that what college is all about? Isn’t that the way to have fun and learn in the process?
The problem is twofold. If you do not prune your activities and courses. Listen to your heart and mind and learn what you love so you will choose the best future path. You have a long life ahead of you at graduation. The other problem is you will be judged by your undergraduate years and an all-you-can-eat approach can hinder you next best steps.
You will be judged at graduation, or before. Your future employers will judge your undergraduate record. You will or will not get into the graduate or professional programs you choose according to your undergraduate performance. To succeed you need to carefully curate your resumé. On it are at most four categories that matter. The first is what classes you took and how you did in them.
First, take as few classes as possible for your single major and get high grades. Remember to choose classes according to requirements. Also, pick good professors over interesting sounding topics. A poor professor can make any topic boring. How to get those high grades comes from few classes and good study habits, a topic for another chapter.
Second, do research in a laboratory. This lets you excel in something academic outside of the classroom. If you are a science major (including social science and engineering of course), it is easy to get into a research lab, work hard, and learn that way. If not science, there are still opportunities for research projects. Talk to your professors. This experience is best done for multiple years. Unless it is a poor fit, stick to one lab.
Third, find a way to help others. It can be in the community with outreach projects in public schools, at a museum, or something similar. It can be on campus also, either tutoring other students, becoming resident associate or whatever your college or university has. It could be related to your major, related to gardening or the environment, but it must be clear that it is a helping activity. It is also best to find one activity and stick to it.
Fourth, there is room for another activity outside the classroom for most students. It could be music, drama, or art. It could be a club of some kind. It should be just for fun, but focus here too is good. There may be five or twenty tempting clubs. Pick one, or try a few your first year and then choose. I feel pretty strongly that if it is an exclusive club, it should only be exclusive based on the nature of the activity. If music, on ability to play the trumpet, for example. Stay away from exclusive clubs, sororities, fraternities, and the like. They can taint.
So, that is it. Choose your undergrad activities as if you were ordering at an elegant restaurant, not a cheap buffet. Limit and focus and you will have good grades, a great letter of recommendation from your research advisor, indications that you care about others, and can have fun. Restraint in these ways is the main thing separating successful happy, well-rested students from the rest and these are the ones with great options afterwards. Oh, and don’t forget to make friends. A few really good friends are the most important. Be there for them and they will be there for you, if they are well-chosen friends. The restraint this approach calls for is also one of the hardest but most rewarding things you will do.