When I visited Hawaii for the first time I went to a hike located near Pearl City called the Waimano loop trail. I found it by accident and it turned out to be one of my favorite hikes in Oahu. The trail itself is really short and you have to go thru the jungle, but right before the waterfall the ground becomes really muddy and inclined.
Once you hit the waterfall you´ll see a pond where you can swim to the waterfall, and in the other side there´s a second pond with a rope that you can use to swing into the water. It is a really fun and different hike that you can do in 20 minutes.
This last Christmas I visited Hawaii and I tried to find this hike again, but I didn't remember the name so when I tried to find it I ended up in a completely different one. It is called the Aiea loop trail, and even if it wasn't what I expected I still enjoyed it. The hike takes about 2 hrs but is not complicated at all. The path is really easy to follow and there is a breath-taking view from one side of the mountain. Also, you can walk to another waterfall but it is not easy to find.
Even though I didn't like this hike as much as Waimano loop it is worth to go for a walk and enjoy the views of H3 from the top. It is also a good hike to do with dogs.