One of the biggest challenges of playing the ukulele is its limited amount of notes. You can’t hit the low notes that guitarists play their riffs around and normally you can’t play as high as a piano, but there are several workarounds.
Artificial harmonics can give you access to some insanely high notes and give them a different ring. Most people don’t play AH any higher than their fretboard and in this case that kind of misses the point. A great example of off the fretboard artificial harmonics is Eric Johnson’s SRV solo – fabulous song. I have already written about how to play artificial harmonics. You can find that article on the “Harmonics” page.
Using a slide is another way to access the highest notes. With a slide, the fretboard doesn’t really exist and you can keep sliding up the strings until you run off the back of the bridge. The notes repeat up the length of the string (even when fretboard’s not there), so keep that in mind as you try and figure out your own riffs. Anything can work as a slide – even your fingernail. Granted it doesn’t have the cleanest sound, but guys like Jake Shimabukuro and Bela Fleck use their fingernails to go beyond the fretboard.
Another option would be a POG (polyphonic octave generator) effects pedal. It takes the incoming signal and adds a sub-octave or octave up. Knobs or sliders control the volume of the extra octaves and the “dry” unaffected signal. So you could turn down the main signal and just have an octave above or below. It’s a bit like cheating, but no one can say Tom Morello didn’t do well with his pedals.