FWIW, highlights of my university experience included playing in various orchestras. If she's likely to be going somewhere with an active musical scene, and likes that idea, then intensively learning an in-demand orchestral instrument might be an excellent idea. (What's in demand varies, but often includes strings, because orchestras need a lot of them!)
The other ideas sound good too though. Does the university have to approve, if it's a gap year after the offer has been made? I don't know how your system works that way. My guess would be, though, that a university might be amenable to gap plans in a young student that they wouldn't approve of in a normal-age one, because they might see merely adding a year of age as a significant benefit in itself.
