What are some options to do over the summer that will help me in grad school?
If you want to do something that will help you get into and succeed in graduate school, I strongly recommend looking for a summer research project with a professor.  Many professors hire students over the summer to work on projects with them, and some institutions even provide internal "student research" support that makes it particularly easy for any professor to do so.

There are many advantages of doing such a project, including:


Getting a taste of research in general and also in a particular area, to see if you really like it
Summer projects let you focus on research in a way that term-time projects do not, yet can still be confined to 9-5 hours.
You will have an opportunity to learn a lot of research-specific skills.
You will get to exercise things you learned in your classes, which may significantly improve both your knowledge and your motivation to learn, since you will tangle with the realities of applying this knowledge, and may improve your future grades.
You will be able to talk about research experience and may be able to get a good recommendation letter from the professor you work with, both of which are extremely valuable for grad school applications.


Now, if you prefer to do other things, self-care, etc., there's no reason that you have to do such a project.  It can, however, be a lovely experience, and if you're choosing between working a research job and working a non-research job, I highly recommend making the research choice!