My family lives in the islands – big island and Maui. I second a lot of what Jazzman says.

For the big island, if you want to be on the ‘dry side’ (leeward), make sure to go north of Waikoloa – you don’t need to go as far north (or inland) as Wailea. The vog (as SO2 is known) can get nasty south of there. I personally really like the Hamakua coast – relatively affordable, rural but not totally removed from the city (Hilo – and I do use ‘city’ in the loosest sense of the word), and absolutely gorgeous. Tsunamis aren’t really a big threat in most places – just very edge of the coastline. My family lives in a rural area south of Hilo. Its so peaceful, and it was cheap (they bought land and built a 3500 sq. ft. house for about $250k). But you have to look at lava flow maps, since a Mauna Loa eruption could wipe out your house if you are in a flow zone.

Kihei is awesome. 80, sunny, low humidity almost every day, not a ton of mosquitos, and one of the more affordable beach cities on Maui (at least when compared to Ka’anapali and Lahaina). There are a lot of condos, relatively less houses.