Amy Y.
Yelp
We stopped by Hoomaluhia Botanical Garden at the beginning of our itinerary around O'ahu's windward (eastern) coast, and only really had about half an hour to spare. It is free to enter, and it's a scenic drive to various viewpoints.
We arrived not too long after they opened, and there were some groups and tours there, but not too many. As we drove further in, we found that there were even less people. There were numerous picture perfect photo captures, and the people who were also taking in the scenery did not linger too long. We saw a small group taking wedding photos with the beautiful mountain ridges in the background. They offer various activities including fishing and camping depending on season.
They apparently have tropical plants from Philippines, Malaysia, Tropical America, India & Sri Lanka, Melanesia, Hawaii, Polynesia, and Africa grouped geographically, although I didn't find the signage very helpful to indicate what was what or where to go. Overall, it was quite tranquil and I was satisfied with our sparse half an hour adventure, having minimal knowledge of where to go. There was a guard when we entered, but we weren't given advice on how to traverse the grounds.
If you want a low key, rather peaceful nature area to go to, this would be a recommendation; the most prominent photo spot has signs telling you to move (drive) along, and while the rest of the botanical garden is pretty, you'd probably prefer photos elsewhere if you wanted to scream "Hawaii" in your vacation photos.