I use an old Microsoft Lumia 735 as an MP3 player. You can pick one up for
$30 on eBay. A nice 4.7" OLED screen, replaceable battery and SD card reader. Plus I really like Windows Phone 10. A shame it didn't catch on. With Google-Fi I'm able to add a data only SIM card so I have mobile data on it without having to pay for an extra line.