First, I'll question the assumption here that what others thought about what they "should" have done is really relevant to you. It really good to ask, but be skeptical of the answers, There will be lots of cognitive bias involved. Lots of monday morning quarterbacking. Lots of selection bias. If you think you want to study physics, and then ask physicists what they wished they'd done, you won't get the perspective of those who switched to biology or journalism. You also may not get very practical, actionable advice. I can say "I wished I'd studied harder, and spent less time backpacking." But that just may not have been a healthy option for me at the time. Maybe I "should have" but that doesn't mean I "could have".
The first couple of years as an undergrad are usually a time of great upheaval, insecurity, learning, and decisions. I'm not sure that looking back we will have a great perspective on the reality we were dealing with at the time.
Now, after the big disclaimer:
My advice is to learn stuff.
Undergrads tend to focus a bit too much on grades at the expense of real education. Grades can be important and can't be ignored, but a few years after you've graduated and moved on, no one really knows or cares what your grades were. They care about what you know. These two things aren't always well correlated. When I was hiring EEs, I honestly didn't really care or even look at what their grades were. I could tell everything I needed to know by asking a few technical questions. There are a lot of reasons you might get a low grade in a course that have nothing to do with how well you know the material a few years later, or vice-versa.
Your grades are important but only have a transient value, what you've really learned will last your whole career.
