repurposing lumia 640's

LOL, during the fire sales I stocked up. I have 2 640 xl's and 3 640's. Great phones but getting dated. As 3 of the 5 phones are still nearly unused and all nearly obsolete, how can I get use out of these phones without using them as phones?
MP3 player comes to mind, they do sound good with my klipsch earphones.

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