It's hard to believe that people want to play an instrument that isn't a ukulele, but some instruments are so close to a ukulele it's understandable, whereas others sound great with a ukulele.
The Portuguese sailors that took their little instrument to Hawaii, also took it to other areas of the world. So, there ukulele-type instruments all over the world like the Cavaquinho in Brazil and the Charango in Bolivia.
There is such a thing as a bass ukulele (take a look at this site) but one alternative is the Ashbory bass. In size, it is closer to a ukulele than a bass guitar and its sound is closer to that of an acoustic bass (which complements the ukulele well).
The Guitarlele, as its name suggests, is half way between a guitar and a uke. It has six strings (low to high like a guitar), but is smaller and is in the same pitch range as the ukulele (the top four strings are tuned like a low-G ukulele).
Uke players, often not being too worried about ridicule, quite often use other novelty instruments. Particularly popular are the kazoo, the melodica and, on occasion, the nose flute.