If you don't want to invest in more MIDI instruments to import into your library, you can always edit the simple instruments already there... which you most likely know. Here's a hint for an instrument often overlooked in our repertoire as we develop new songs: The old-fashioned harpsichord. You know, Bach and all.
To get a twangy harpsichord sound,
But this is easier: Use "Clean Electric" or some similar one-string-plucking guitar MIDI instrument. Play simple, somewhat slow arpeggios of chords. The final sound does sound much like an old harpsichord.