Mp3 compatible with Spotify

[Forgothowmuchlhatehomeschoolin]

Posting here for traffic so lcan ask the wise ladies in MN for a recommendations please.
Dd9 wants an mp3 player for when she goes running so needs to be small and light - we have got Spotify but the one we bought off Amazon says it is compatible but took ages to try and convert the playlists and ended up giving up and sending it back.
Didn't want to spend loads really - any suggestions please? Thanks in advance.