Waianae Coast, Oahu
Be kind to the locals, and clean up your mess and you will have a great time here. I've done many adventures here including dolpin diving. I write about a lot of these activities on my fun blog www.captaincrunches.com .
The Waianae Coast is the far western side of Oahu, and it looks inviting on a map. Unfortunately, it is the most impoverished, poorly-cared-for, junkiest part of Oahu and is only worth visiting if you've seen everywhere else on the island (and some of those places many times over). Most of the beaches on this coast have been taken over by camps of homeless people and crystal meth users, and are decidely unsafe. That being said, if you dive far enough toward the end of Farmington Road, you will be treated to some nice, safe beaches and beautiful scenary -- though nice beaches and beautiful scenary can be found on almost every othe part of the island as well.
Unique Suggestions: If you are going to drive Farmington Road, make sure you go to the end -- otherwise the trip will have been a waste. It ends in a beautiful state park with a nice beach that is crowded enough that you probably don't have to worry about your car being looted. The sea cliffs are spectacular and the sea gets the evening sun, so this is an enjoyable place.
Fun Alternatives: The North Shore and Kailua/Lanikai are much better alternatives if you want to hit some beaches outside of Honolulu.