I got this little iPod nano-shaped Chinese Mp3 player[1]. It plays music and uses .m3u playlist files that are easy enough to edit on your PC. You can even set bookmarks in MP3 files so if you have a large audiobook file, you can save your place in it. It has some idiosyncrasies (such as not playing songs in order by default; the alphabetization is weird) but the killer features are: it's small, and I have literally never once run out the battery. I do charge it occasionally, but even after several hours of hiking, the battery indicator is at 3/3 bars.
It takes a micro SD card so you can load music however you want and then just plug the SD card in
[1] https://www.amazon.com/gp/product/B01LZVDMEL/ref=ppx_yo_dt_b...