I believe the instrumental case was absorbed by the ablative in Latin and by the dative in Greek. Is there any way at all in which influence of the old instrumental can be seen in Latin or Greek?—outside the bare fact that the ablative and dative, respectively, can be used to indicate an instrument.
Any remains in morphology, in semantic oddities, in syntactic sequences, etc., or whatever you can think of, would be of interest. I know very little of the instrumental case, so I don't really know what to look for. Of course "no" is also a valid answer. (For comparison, I believe the typical vowel i of the Proto-Indo-European optative mood is in some way reflected in the Latin subjunctive.) [Additional example: the way dual endings survive in words like frenī, suggested by Draconis.]