Playing a ballad, nice and slow, using only sustained notes (no tremolo) teaches you many things. First how to hold a clear and sustained note. Leaving you fingers down long after you have struck the string. Your mandolin can a make beautiful sound, make sure you are doing it justice by holding each note for as long as possible. As you do this notice how clean your sound becomes. Listen for any buzzes. These can be due to lack of pressure from the left hand, positioning the left hand finger too far back on the fret or too far forward on the fret itself. Just behind the fret gives the greatest clarity and least pressure required. If you use too much pressure with the left hand you may find you are pulling the strings out of line or bending them causing them to go sharp. When you have checked your left hand position turn you attention to your right hand and the plectrum. Is your down stroke producing a good volume and clean attack. Are you angling the plectrum so you are striking both strings of the pair to maximise resonance and overtones? if your next note is on another string can you hold the one you are playing as you strike the next allowing two notes to sound in harmony together? Paying attention to each of these elements will improve your tone, intonation, clarity and quality of sound and sustain.
Two good ballads to play are ‘Autumn Leaves’ for a swing ballad or ‘Black Orpheus’ for a Latin ballad.