Reddit mentions of The Hikers Guide to O‘ahu: Updated and Expanded

This blog has some good hikes and ranks them on difficulty: http://www.unrealhawaii.com/

This book is also fantastic and helped me find and navigate a bunch of hikes when I was learning the trails: https://www.amazon.com/Hikers-Guide-Oahu-Updated-Expanded/dp/0824838998

You want views, you gotta work them (mostly):

• Pauoa Flats via Kalawahine will get you to an overlook over Nuuanu valley and this hike isn't too difficult because you start up on the ridge.
  
• Kuliuouou requires some effort, but both the hike and view are amazing. Bring lots of water and snacks.
  
• Wiliwilinui is probably the shortest hike with the best view. One big up. It's in a gated community, limited parking, get there early.
  
• Hawaii Loa is the ridge directly east of Wiliwilinui, similar view, a bit more difficult, also a gated community.
  
• Diamond Head (I know, I know), but its an easy hike with a nice view of the South shore.
  
• Mt. Olympus will give great views of Manoa and Palolo. Best to turn around when the sanctioned trail ends, but you can continue on the the summit. Though this can be a bit sketch. Turn back if you don't trust yourself.
  
• Lanipo is a LONG hike, but it does eventually summit.
  
• Aiea Loop is a nice trail that has a great view above H3 at around the halfway point.
  
• Koko Head climbs up old railway tracks and provides a nice view of the south east side.
  
• Puu O Hulu if you find yourself on the west side. Quick steep up, but great views of Waianae valley and the west side.
  
  If you're trying a new hike, don't hike alone. If you do decide to hike alone, let someone else know your plans. Always bring water (2-3 liters for the hikes mentioned above) and some snacks. Also, bring your cell phone.