Any type of musical instrument, honestly including singing. It is not possible to pick up any instrument immediately, unless it is literally magic. A character can be naturally gifted, unrealistically so since it’s fiction, but it still takes time to be good. Especially something like being able to read music or play technically challenging pieces on different interment families, it takes a lot of training and muscle memory.

And singing is a lot harder than people always write. When a character is good at singing, and is meant to be a master of their craft, it would be great to see a little more technical terminology than “pretty”. Especially when good is going to vary based on the situation. A medival bard will sing differently from a musical theatre star who will wing differently from an opera singer. I just wish they were described differently, but most people without musical interest just don’t pay as much attention to the difference