MP3 players aren't nearly as popular as they used to be, but Instructables user BrittLiv's MP3 player-inside-of-a-cassette is cool enough that it might be worth making regardless.
The build here uses an old cassette, a basic USB MP3 player, a MicroSD card, and an assortment of cheap materials. You're essentially taking the cassette apart, stuffing a bunch of different materials into the cassette, then installing a set of buttons so you can control the player. The end result is a custom MP3 player that plays songs from the MicroSD in a linear fashion, just like an old cassette.
Cassette MP3 Player [Instructables]