Ukulele, Guitar and accordion are all polyphonic. Therefore a straight specialist melodic instrument is called for. Either Violin, Sax, Flute etc. Just be careful of their egos. For some strange reason single note instrumentalists think that the entire world revolves around them.
I think that Celtic harp goes very well with 'ukulele, when playing some of the beautiful Hawaiian tunes, as well as many other tunes. I also play accordion and know from experience that 'ukulele and mandolin are softer sounding instruments. An accordion is a much louder instrument, so the player must take that into consideration and not play too loud.
I like to bring my Tuba (or my Sousaphone) or as I like to call it the Contrabass Wind Ukulele to uke club. Check out a video here:http://www.youtube.com/watch?v=1qu3GiCj3yQ