You won't impress the adcoms under the H&A section. Neither will most of the traditional students either unless it's something bigger like a Rhodes scholarship or other big national award. Put your undergrad stuff there just so there's filler, but nobody reviewing your application will spend much time on that part. Put stuff like any scholarships you had, whatever lists you were on, etc. If you have any adult awards, mention those too, but it's ok if nothing adulty really fits there.
As for the best use of your time, hard to say without knowing more about you and your strengths/weaknesses overall. Obviously you could benefit from using the time to bolster any flimsy parts of your app. Research is not an essential part of the app like some other things, but non trads frequently lack this item and if you have time and opportunity, might be worth getting involved with some project. I'm sure there's some QI project related to nursing you could pitch in with as well and maybe even get your name on a paper. That's just if you have everything else covered already; totally not essential, just checking another box.
Looking at the date/time on my screen, I do wonder what your timeline is. If your plan is to take the MCAT asap and apply this cycle, maybe try starting a few things prior to submission so you can include them and then keep them going or let them fall by the wayside depending on how your year goes. I'd say focus most of your efforts on making a living and turning applications around ASAP. Keep other things marinating just enough in case you have to re-apply and need to say something about how you spent your year, but as a real adult I think everyone expects you would have spent the bulk of your time working and living life.
I think with a healthcare background and assuming you've done a little shadowing and do some occasional volunteering, you should be set from that side of things. I can't imagine someone looking at an application from a working RN and thinking they aren't cut out for medicine because they don't know what it's like. Honestly, the biggest mistake you could make would be to apply too late in the cycle.
Remember that while being a non-trad doesn't excuse us from needing solid numbers and checking all the same boxes the traditional students do, it does grant us a little leeway assuming the groundwork is solid. My shadowing and volunteering hours were nothing compared to some I see posted here, but it wasn't remotely an issue for me. I had enough to convince people I knew what I was getting myself into and would stick with it; as a nurse you clearly have that covered too. Overall, keep adulting and find ways to distinguish yourself as a successful professional in your field who is also involved in giving back to the community.
