Getting to Hawaii
All the major airports are very easy to bike in and out of, though if you are lugging around a bike box that is a different matter. There are good ground transport options (public bus and shuttle) between the Honolulu airport and Waikiki. For the other islands, taxi or car rental are your only motorized options if your hotel (or campground) does not have shuttle service to the airport.

Getting Around
The island of Oahu is served by a surprisingly good bus system. Buses are equipped with bike racks, and there are several express routes to quickly get you from one end of the island to the other. The other islands are quite rural, which limit your travel options to biking, walking, swimming, taxi, or car rental. For interisland travel, you can make a quick hop on Hawaiian Airlines or Aloha airlines. Plans for a comprehensive ferry system are in the works, which should make things much more convenient for cyclists when it begins service.

Weather
Except for the occassional hurricane, you can of course expect perfect tropical weather year-round. There is considerable variation in precipitation between "wet" and "dry" side of the islands, as well as considerable decline in temperature with altitude -- it is possible to bike from tropical beaches up to snow-capped peaks.

Route Planning
Because the islands are so tiny, there is no need to bring panier bags -- pick a central location and plan day trips from there. On Oahu, you can rely on "TheBus" to get you back to your starting point. On the other islands, plan a loop ride or hitch a ride. Pretty much any road atlas can be used to plan an iterary as there are not a lot of roads to choose from. Generally, there will only be one road going your way, and in some cases it might not even be paved. For cyclists comfortable riding in traffic, almost every road and highway is bikable.