Top Picks: Ho‘omaluhia Botanical Garden (Educational, Koi Pond, Wildlife Sightings, Windward)
Just try to keep your jaw from dropping as you drive into this utterly amazing and free green space (45-680 Luluku Road, Kaneohe, HI 96744). 
Great parking lot and visitors center here, pick up a map and start going. Right near the visitors center there is a trail with tons of exotic looking labeled plants including some cocoa beans! 
We took the path down towards the lake after talking to some ecstatic man who told us it was beyond beautiful. 
Agreed, I was gushing too. After the short walk down there we saw tons of mongoose, an invasive species from India, who happen to be adorably frisky looking. 
There were also a lot of waterfowl including heron, ducks, and a specifically interesting duck the Hawaiian coot (black one). 
The scene of the lake against the mountain was beautiful, and when approaching the bank you can’t help but notice the fiery color just under the surface. 
Tons of koi are here and I wish we brought some bread to feed them it would have been a show. On weekends the park provides bamboo poles and you can fish for free (catch and release). We took a trail over a little bridge to continue walking along the edge of the water. We saw even more beautiful flora and lots of great birds; this part of the park was very quiet. 
The trail kind of fizzles out into the woods, there are trails that get really small and might just be boar trails. We entered unknowingly and didn’t encounter any boar, but I did get eaten alive by mosquitoes. Bring repellent! We headed back the way we came and wandered to the very quiet sections of the park which were dedicated to plants from a specific area. These are all connected by a road and have their own parking lots which is nice if you don’t want to walk the whole thing considering the park is pretty big. Along this road we walked up a big hill which led us to an amazing panoramic view of the crater we were in, and some of the science behind it. 
We also saw a ruin of an old oven which produced charcoal from guava trees. There so are many trails and this is such a huge park you could really spend a lot of time here. I couldn’t recommend coming here enough, it is such an enchanted setting.