What is “most relevant” is your experience as a researcher. Of course, your masters course should be included, especially since this is how you know your referees. However, in this respect, summer internships may be more relevant than other activities on your resume. They can show relevant experience in industry or a research environment.
If you did menial jobs during the summer, this may not be as relevant. If you can include it, it may still help to explain the “gap” in your resume and why you weren’t involved in research or volunteer activities at the time. We can’t all afford to live without income for an extended period of time. You should not be disadvantaged for this and it arguably shows your work ethic as well.
Of course, any way to show that you’ve had interests relevant to a research career helps but is not essential. It is understood that many students only realise that a research career is suitable for them once they’ve experienced a research environment. That is part of the purpose of summer internships afterall. Many first generation graduate students do not know what academia is like until they experience it and aren’t expected to. There are supervisors who are understanding about this (although not all of them are unfortunately).
I no longer include my non-research jobs on my resume, unless they demonstrate experience in teaching, academic writing, or similar skills. I don’t mention awards (everyone gets them anyways) unless they held a monetary value such as a competitive scholarship. Competing for grant funding is a major part of an academic career any experience at securing funds on your own is worth including.
However, as a postdoc, I still include my summer internships. These establish that I’ve had a long relationship with my previous supervisors and maintained a good relationship. It shows that I’ve changed research topic and managed to learn the necessary new techniques for the new field. It shows that I can work in different environments on my own and in a larger collaborative team. Most importantly, my summer internship project ended up being published which demonstrates that I can as see a project through to completion. As such, these positions are relevant to your application but you should think about their purpose before including them. They will not spend much time reading your resume so everything on it should be there for a reason.
Of course, every country and every institution has different requirements. Your experience may vary. Do pay attention to the guidelines of the institution and if possible contact a prospective supervisor for their support and heed their advice since they should know the system.