To my ear, the octave uke sounds a bit like a viola -- with deeply resonant notes. I've been exploring this idea in these YouTube videos below. An octave ukulele is a baritone uke, strung with octave strings. It is a full octave lower than a tenor ukulele. Ukes are affordable, portable instruments which are a perfect first instrument for children -- or adults. I'm particularly fond of the Makala Baritone uke, which sells new for $99. Buy it at your local music store, when possible.
My current octave uke is this one.
If you live in a Habitat for Humanity house, I sometimes teach music lessons for free. I prefer if you organize a small group of students, to make my time use the most worthwhile.