If this is your first time, suggest Maui

Maui has more activities that suit most tourists' taste - some may complain it is "too commercialized" as well. - that is the draw back. Southern part of Maui (the sunny side) has some spetacular beaches. Of course Haleakkara (spelling?) NP is a must visit too.

Hawaii (Big Island) has the active volcano and where else in the world you can drive to the edge of a volcano and not risking your life?! This is the biggest island in the Hawaii Island Chain. It takes a very very long day to drive the whole loop - better approach is to split the stay between Hilo and Kona (or Waikoloa). Along the road to Hilo has many orchid farms well worth touring.

Kauai is quite rustic and does not have nice beaches like Maui because of the river. The island is nicknamed to Garden Isle so you can imagine how it looks like.

Inter-Island flights now are much cheaper due to competition brought by a new comer. From late June onward the price come down about 50% - good for your time frame.