Plugins>apps>lastfm_scrobbler
when you run it the first time it'll ask you to 'enable playback logging' select YES
and optionally set your options, cick cancel and if asked select yes to save changes
go play some tracks at some time later in the future run the scrobbler plugin again and select Export
you now have your log its located at /.scrobbler.log
Note: you don't have to run the scrobbler plugin except when you want to update the log
but do be aware that you should do it occasionally once playback logging is active
since it will dump rockbox's internal log and mitigate performance issues trying to load such a large log
I've had no ill effect with a playback log as big as 10mb but its something to be aware of if you let it go for months..