I would highly recommend taking classes in disciplines that you are unlikely to learn about in your desired field of study of mechanical engineering. Classes that I recall now that I'm 16 years removed: Intro to Anthropology Intro to photojournalism Micro/Macro Economics Poetry Race and Minorities Introduction to Drama Fundamentals of Speech These are all courses that have stuck with me over the years. Good luck! Enjoy the journey.
I came here to plug liberal studies classes like these. One of my favorite classes from my undergrad was an American History Gen. Ed. called "Social Forces That Shaped America". We delved into all kinds of issues and having the backdrop of American history to work off of helped structure the class in a way that got me thinking very hard about everything I had previously learned.
One of the reasons why I think it is important to get a taste of completely other fields of study is that you are introduced to completely different ways to think. What I've noticed with the friends from high school that I still regularly talk to is that the way of thinking taught in their field of study permeates into their regular thinking. 'Looking over the fence' helped me better understand how I think and how they think.
In my experience, the most innovative, groundbreaking research tends to come from people with background in multiple fields. Taking insights from one discipline and bringing them into another is so important.One of the reasons why I think it is important to get a taste of completely other fields of study is that you are introduced to completely different ways to think.