It's actually called Puu Ualakaa State Park, but nobody calls it that. Locals just refer to it as "Tantalus" since it's on Tantalus Mountain and it gives you the best panoramic view of Diamond Head, Waikiki, downtown Honolulu, Punchbowl, and Pearl Harbor. It feels like you can see eternity from here.
It's a long and winding road to the park entrance. Once you get to the gate, you have to follow the road for a little longer until you get to one of two parking lots. You can park at the first parking lot, that has shaded benches and a restroom or drive down to the second parking lot.
The second parking lot is right next to the lookout. You might see photography groups up here taking pictures of Honolulu or Diamond Head.
And when you get to the lookout, you get this view. To the left or southeast is a great view of Diamond Head and Waikiki.
To the front (south) is a great view of downtown Honolulu.
And finally, to the right or southwest is Punchbowl and Pearl Harbor. That's pretty much it for this lookout, the park itself is small and not many hiking trails. Most people aren't here for the trails, but for the view. However, since you're up here you might as well swing by Manoa Valley and check out one of the historic sites in Honolulu: the Wai'oli Tea Room.
The Wai'oli Tea Room has been around since 1922 and it's located at 2950 Manoa Road, which is about 15 to 20 minutes away from the Tantalus Lookout. I've tried lunch and the I think the food is just okay. I haven't tried the tea and snacks, though I'd recommend it since it's their specialty. They serve breakfast, lunch, and dinner as well as tea. Tea time is from 10:30 am to 3:30 pm.