As far as personal experience is concerned, the summer vacation after the college entrance examination is the freest holiday. You don't have to attend numerous secondary school cram schools as before, and you don't have to think about the future. You will practice or take exams in the university in the future. Be sure to take the time to do something you've always wanted to do but haven't done. For example, I always wanted to learn German, but I did something else in the summer vacation after the college entrance examination. It's a pity that I don't have such a good chance to learn German now. Or if you like traveling, you can take this opportunity to go out for a walk.
On the other hand, we should also make some preparations for college life.
As said upstairs, many universities have all kinds of exams at the beginning of school. You might as well ask your brothers and sisters in the university you entered and make some preparations in advance. Anything in front always wins the first chance, right? So the accumulated time can help you do more things than others in college, and that's how the advantage comes out.
Personally, I don't agree to spend most of my time on part-time jobs, because you don't have any ability to complete technical work now. If it's pure manual labor, it's not bad to experience life and increase experience, but the opportunity cost is a bit high, so you should focus on preparing for the future.