Any suggestions for growing a fast tall hedge in Waialua in the sun that could be pretty. too?
Hibiscus acetosella is my number one recommendation.
Maybe it's just that I like it so much and it could fit in this situation, lol.
It will be sun-hardy once established (seedlings may need some protection until they're about a foot or so, give it a few weeks) and looks like Red Japanese Maple. With the addition of the red Hib. flowers of course. The stuff grows like mad, a seedling grew to 4' in 1.5 months for me. Once it gets that tall, you can top it off and keep trimming it so that it will fill out and become more of a solid hedge.
Here is a link that might be useful: Google Images 'Hibisus Aetosella'
So many possible answers to that question! How much space can you give the hedge? By 'pretty' do you mean flowering or just nice to look at? How tall do you need it to get? How soon does it need to get to that size?
Bamboo is fast. So is Variegated Hau. Duranta is fairly quick and bears those beautiful blue flowers and golden berries at the same time. Even a row of bananas might work. A'alii is moderately fast and a native. Since you mention trees, Glircidia might be another option.
Keep in mind that how fast the hedge grows is usually a pretty good indicator of how much maintenance will be required to keep it in bounds later. Don't ask me how i know ; )