? First, English. If I regret not learning anything well when I grow up, I must speak English. Because English is not only a subject, but also the most easily used thing in our daily life, you may not have any chance to work out a math problem on the spot, but you will have countless opportunities to have an English conversation with your mouth open. At this time, if English is arrogant, it should be something proud. Therefore, I suggest that college students must teach themselves English during the summer vacation.
English is a thing that practice makes perfect. As long as you read, listen and write every day, you will make rapid progress. So don't waste your summer vacation. Make good use of every day to learn a foreign language. I believe it will benefit you for life.
Second, subjects related to your major. College students can use their summer vacation time to teach themselves something related to their major, which can be professional books or related magazines and newspapers, so that they can learn faster and better than their classmates when they start school. It takes time to accumulate knowledge, so every day's efforts will always pay off.
Third, what you are interested in. Besides knowledge, you can also teach yourself something you are interested in during the summer vacation, such as basketball, flower arranging or painting. Now online tutorials are so developed that you can use online videos to teach yourself. Even if you can't do it well, at least you can.