Lyon Arboretum, tucked away at the end of Manoa Valley, is such a gem. I had never really explored the length of this 200 acre garden before, and I also haven't visited it for years.
Recently, a friend and I decided to take the entire self-guided tour, and what a treat that was. We got lost a little bit and started off-roading in the unmanicured rainforest, but overall, it was a well-spent two hours.
If you go, I strongly advise long pants/ long sleeves due to the heavy mosquitoes, a hat, sunglasses, hiking boots, water, and a snack. Treat this like a hike in the mountains; there aren't bathrooms at regular intervals.
Minutes from civilization but it feels like a world away: beautiful!
After we worked up a huge appetite, we ate at Treetops Restaurant in the old Paradise Park. You would never know it's there because there's one really bad sign that just says "Parking for restaurant only" or something.
You'll know you're there because you'll see a bunch of people and cars, and another lot with a guy in a lawn chair sitting out front monitoring payments for parking. I think those people are going hiking at Manoa Falls. Drive the other way and you'll find Treetops. It's good food at at good price!
Hope you have a good half day doing this!