I agree with learning cover tunes. i have gone off on kicks with certain artists and learned different artists use different keys to suit their voice. Bob Seeger G and C, Petty A etc As I learned each song with each artist i picked up song structure, learned key transitions, variations to minor chords for bridges etc. With Clapton I learned how to integrate a base line into the chord progression.

All this keeps it fresh. Play on Grasshopper!!!

R1