Honestly, one of your best bets is just an old iPod Touch. This isn't what anyone wants to hear, but let me make the case.
I'll acknowledge that adding your own MP3 or lossless files to an iPod Touch is moving towards impossible in 2024, since Apple is steadily working to shutter iTunes. If you want to listen to your original music or deep-cut live recordings that aren't on streaming services, you're shit out of luck. But, if you just want the general experience of an MP3 player, we can make that work.
Let me address the audiophiles here. These days, streaming services like Apple Music's, Qobuz, and Tidal offer lossless streamed music, so there's no reason to prioritize FLAC files from your computer anymore. If you want to get into the hobbyist aspect of it, go for it. But, don't feel like you need to.
The real reason I'm suggesting an iPod Touch is the convenience. Maybe you want to download all your music onto your computer and move it to your player. But honestly, that's a pain in the ass process you can leave behind. With an old iPhone, you can connect to Wi-Fi when you need, download (lossless!) music from a streaming platform, and listen to it distraction free when you're out in the world. It's the best of both worlds. If you want this to be a music player for a kid, Apple's got a comprehensive set of parental controls.
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