John plays a lot (though not all) of his music in "taropatch" (also called open G) tuning on his guitar, DGDGBD. The equivalent on the ukulele would be GCEG (open C tuning). This is best if you use a low G string.
Led Kaapana uses this tuning to play some tunes on his uke, notably "Opihi Moemoe". He also uses the standard GCEA tuning a lot.
Mark Nelson has a nice "slack key" ukulele method book available at
http://www.mark-o.com/