Because it is in the tropics, the weather doesn't vary too much at all - mostly around mid-20s C by day and doesn't drop much at night.
Its a hell of a long way to go for anything less than 2 weeks.
About 70% of all visitors (mostly Americans) go only to Oahu and most of those stay close to Waikoloa Beach. There's plenty of other places to go - geological, flora mainly.
We preferred Kaiui, Big Island and Maui to Oahu - far less touristy.
Not like the US to drive in - no freeways and road widths somewhat similar to UK A roads.