Find a Sansa Clip, Clip+ or similar on ebay and install rockbox on it. Once setup you can just plug it into your computer and drag & drop mp3s like any other USB thumb drive. They're tiny and it's great having physical buttons; you can pause, skip, etc without looking at the device.

There are a bunch more supported players but I've only used Sansa. See: https://www.rockbox.org/wiki/TargetStatus