There a few ways that I'm aware of, but they're all a combination of Research, Extracurriculars, and Electives. Every 5th year should have electives - a crucial difference between graduating and trying to match the year after as a "previous year grad", and doing a 5th year and then entering the match as an "unmatched 5th year", is that as a student you get insurance coverage through the school for electives to be arranged. Plus outside of everything else, logically a school would prefer you to have recent clinical experience and not be too rusty in practice by the time you're going to be a resident. 1. You can follow what the old de-facto route was, which was having a research project to work on during the year (organized at least partially by yourself), in addition to elective time similar to 4th year to further hone clinical skills and get reference letters. This adds to the CV, adds/substantiates research portion of application, provides for academic networking through conferences etc., in addition to things to discuss during interviews and personal statements etc. I think this is a great option if you have previous research or an interest in exploring research

2. Foregoing research and just doing electives. This can maximize your amount of letters, clinical experiences which result in experience/interview examples, and to get a great breadth of exposure into the fields you're considering that will be a part of the rest of your life. You have a whole year so you have A LOT of time. If the extra time is not going towards research, you should try to do other things like making your face known at conferences, leadership, charity work etc. something that can show you're using your time well, and doing things you enjoy can also be a good investment too (wellness related hobbies that can be noted on the app like Yoga cert).

I am not a research person but think that the flexibility fo 5th year is so great, that you can do all of research, electives, extracurriculars, and networking at a pretty sustainable level. My advice would to choose a few things you are really interested in (and may carry that forward through your career) and delve into those, rather than a jack of all trades approach. I did the latter, and there is only so many experiences and components you can talk about in the Statement or Interview, so it's diminishing returns when it comes to the length of your resume. Your mileage may vary of course, but that's what I'd suggest. Excited to hear other opinions!