Watch the Week 5: Melodies on the Fly online guitar lesson by Roberto Dalla Vecchia from Flat Out Flatpicking
Choose a key first, then play a very famous melody on the fly at a slow tempo. Try your best not to make too many mistakes. Choose a different melody every day and test your ability to play what you hear in your head.
Simple melodies are: "Frere Jacques", "Twinkle, Twinkle, Little Star", "Clementine", "London Bridge is Falling Down", "Oh Susanna", "Obladi Oblada", "Bye Bye Love".
Take this a step further, play the same melody in the same key all over the fretboard using different scale positions.