There's an instrument out there that bears a serious look. It's the size of a tenor ukulele, but it has six strings, like a guitar. It's tuned up a fourth from a guitar (like a guitar with a capo on the fifth fret), and it goes by the name of "guitalele" (preferred) or "guitarlele." It's sort of a hybrid.

As to whether one can play serious music on one, give these a listen:

Lute piece by John Dowland .

Lagrima, by Tarrega (I play this one on the classic guitar).

O'Carolan's Receipt. Sounds almost like a lap harp.

The Yamaha GL-1 guitalele is a very nice instrument, well made, and they sell for about $100.00. I have a friend who owns one and she gets a lot of music out of it. I would have one myself, except that my bratwurst-sized fingers find the fingerboard a bit cramped. My fault, not the instrument's. Nancy has no problem with hers.

Don Firth