I get what you're thinking of with having a separate device, but I've found it better to just have a decent extra battery pack to travel with. You get a lot more flexibility with use. Maybe you just wanted to listen to music. Maybe you later decided you actually wanted to read instead, or play a game, or watch a video. Maybe you forgot to sync those podcasts until you got to the airport waiting on the plane and only just downloaded it and now it's a hassle to bust out a cable and sync it to your offline only MP3 player.

Essentially, you could carry a small battery attached to an extra sigle-task dedicated device, or a slightly larger battery in about the same-ish form factor that will let you use that energy to do anything you want with your other device you're probably already carrying.

If your phone is in whatever airplane mode, battery saver mode, etc. it's not going to use that much power just to play local music.