The Arch (Natural Attraction, Windward)
If you park at the Halona Blowhole parking lot (Kalanianaole Highway heading east in Honolulu) and walk across the highway, head west past where the rail ends, you’ll be heading in the right direction. You can pick a comfortable spot to scale the rocks and an obvious trail will appear. The flora here is really interesting full of cacti and their odd star-flowers. 
In the distance you will see the natural arch, a peculiar formation. On the day we visited it was violently windy, and we were already tired from hiking Koko Head and the Koko Crater Botanical Gardens, so we decided against hiking the steep arch and instead admired it. 
How often does something like this happen? If you attempt to hike the arch you might want to wear hiking gloves as it is very tough rock. Free to hike to, and an interesting sight!
Halona Blowhole (Natural Attraction, Windward)
The blowhole is actually a pretty popular attraction where tour buses visit, however it is free and if you’ve never seen a blowhole it’s definitely cool to view for a short time. Just wait for a huge wave and poof! Water comes rushing upwards like a geyser. 
We saw one in Maui for the first time and got really excited by it.
Japanese Fishing Shrine (Cultural, Windward)
Right next door to the blowhole (and on the way to the arch) are some black lava rock stairs that lead to a ‘Jizo’ or Buddhist guardian. 
It is a religious statue left there to protect fishermen and surrounded by offerings. A quick, free, and interesting religious site.