I am a lifelong resident of Hawaii Island, but I had not been here until about 5 years ago. Just outside of Honokaa, up a winding rural road is a slice of paradise (for some of us). Giant ginger plants and other interesting flora make for an nice walk around the open areas of the park. There is a .7 mile nature walk on the far side of the park. The walk is in the forest, but it's flat and I would consider it an easy hike - just one notch above the walk to Yosemite falls from the road if you've ever been. If you haven't it's a 1/4 mile on sidewalk. :-)
For me the highlight is the huge, ancient Ohia trees. I say ancient, they are probably about 400 years old. I can't post pics right now, but check this website out. The tree that he's standing in front of gives you an example of these big trees. http://www.mauiguild.com/Woods/ohia.html. It's humbling to stand in next to such large, old trees.
I have 1-2 hours as the recommended visit. You could spend a little more or a little less depending on how fast you walk. The road to the top gets narrow and you'll need to be cautious and courteous on the way up and down. If you enjoy being in nature this is a stop you should make. Get an early start and plan to stop on your way from Hilo to Kona (or vice versa). You'll be in good time to stop at Tex's Drive-in for some of those famous malasadas.
BTW, there was no cell phone network up there the last time I went. I think that's amazing, but in case you get connectivity anxiety, there is a safety phone (landline) in the pavilion where you park.
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