One key statement in your specific question is seeking summer internships. When applying for internships, it is typically assumed that the applicant does not have direct experience performing those job functions, as that is usually the primary purpose of an internship: to provide experience performing new job functions/working in new environments.
To "market" yourself for internships which will give you direct experience using skills which have greatly increased through recent coursework, the prospective intern can start by saying just that! Begin by ensuring that education information is specific enough on the resume to show specific advanced coursework in these areas (a bullet point listing course names under your education usually suffices). Then, in the cover letter, spend a good few sentences near the top describing just what you want to do in your internship, and why you are prepared to begin learning "on the job," now that you have completed XYZ coursework.
One additional note specific to your question -- I wouldn't tell any prospective internship sponsors that you're not actually looking for a career in the field in which you wish to intern. More likely than not, they would consider you to be wasting their time spent grooming of potential future employees. Many internships are set up so that the intern has a clear path toward obtaining an actual position with the company after the internship and coursework is over, and thus the company isn't just filling slots when they interview potential interns -- they're making an investment in a potential future employee.