Fortunately, Hawaii's volcanic peaks attract plenty of moisture--up to forty feet of rain yearly at Kauai's Mount Waialeale, the rainiest place on the planet. Straying from the boardwalk through the Alakai Swamp (pictured), which is next to Waialeale, is a risky, often fatal proposition. Even Kauai's most experienced outdoorsmen know better than to be caught in a swamp rainstorm, usually accompanied by dense fog.
But most of the 149 other kinds of rain are not nearly as foreboding. Walking in a tropical garden in the rain is one of the best experiences to be had. Bring a waterproof shell and get into it. Here are a few of the rains you may encounter:

ua lanipili: downpour lasting several days
ua awa: cold bitter rain
he ua lanipali: heavy shower pakapaka: large, spattering drops
awa awa: fine rain that's cold
koiawe: light, moving shower
hookili: fine gentle rain, a form much loved
noe kolo: small, fine mountain rain that mixes with the thicker rain of the forests
You'll find directions to the Alakai Swamp trail in the Kauai Trailblazer guidebook.