It depends how much of a contrast you are looking for. If you want a more classical stringed instrument, I would reccommend either the violin, cello, viola, or bass.
As for a more typical country style stringed instrument similar to the guitar, I would suggest some more interesting instruments such as the banjo, mandolin, or veena.
The harp would also work, too!
The question is, have you ever played bass guitar? It is a very fun instrument to play and sounds amazing if you can play it right.
