I came back to guitar after being self-taught and not very good, and was pleasantly surprised that the Trinity Guitar Initial Grade (eg the first book you'd start a kid on) had some awesome music that you could make the melodies of very quickly. Like, 2-3 hours of practice and you can slowly pick out the melody from Runaway Train, Orange Crush, and Where did you sleep last night.

I guess my point is you can get some good sounds out of the guitar very quickly, which can be very intrinsically motivating.