A song that you can play on an instrument.
Hm. I did singing as my main instrument for Standard Grade music. Yes, you can do that. I sang I Could Have Danced All Night, and some other things.
I have tried to learn a few instruments in my time - piano, violin and recorder, to name three - with varying degrees of success. These days I can usually pick out melodies by ear on a piano or a keyboard, but not with much proficiency. One I used to do quite a lot, for no real reason that I can remember, was Over the Rainbow.