How about a Singing Sword? +1 Greatsword, made of silver, that's sings loudly when drawn. Originally gave a "moral boost", especially against things affecting the mind, so we could just interpretations that as advantage against mind affecting spells and effects, but only while drawn. I think it'd be appropriate for a Bard.
Out of things that are interesting in the DM Guide, the Sword of Sharpness is awesome, or a sword of wounding.
And i I could always just go with a typical +2 Greatsword. Those are always reliable.
Sorry. Just shopping around.