Dole Plantation is cool, but shouldn't take too long. If you're driving around the island to check out the North Shore and Haleiwa town, you can take Kamehameha ("Kam") Highway down through central Oahu and see fields and fields of pineapple. We'll stop by for the visitor center and to get some Dole Whip.
PCC. That's before the North Shore if you're taking Kam Hwy counter-clockwise. It's kind of a World Showcase of the Polynesian islands, their people, and culture. If you plan to do the luau, it'll be an all day thing, and they're supposed to have a pretty cool nighttime show. However, it's been years since I visited.
Since you're looking at being in the Laie/Kahuku area (where the PCC is), shrimp trucks are a big thing up there. Think plates of juicy, garlicky shrimp scampi. The two most popular are Giovanni's and Romy's. While it's an experience and the food is tasty, be prepared to wait. These places have been featured on too many foodie shows and blogs that they're now swamped by tourists all the time. Last December, we made a stop at Giovanni's and waited an hour for our food.
Similarly, in Haleiwa, there's Matsumoto's shave ice, which you'll find on a lot of foodie shows and blogs. Adam Sandler wore their t-shirt in his movie 50 First Dates. Years ago, my wife and I saw Ted Danson, Mary Steenbergen and family there (and I think scared them away by pointing them out). But since it's become so popular, there's often a line out the door. If you don't want to wait, just snap a pic with their sign, and get some shave ice at Aulani (where you can add on some Mickey ears).
In between Kahuku and Haleiwa is Laniakea Beach, where you can usually find sea turtles lounging on the beach. Find a spot on the side of the road to park, and head over there. There's usually some conservancy volunteer watching out for and educating people about the turtles.
Generally, if you're going to make a trip up to Laie/Kahuku/North Shore, plan for a weekday during non-commute hours. The traffic will be better and there should be lower crowds. Allow extra time because there are a lot of stretches of Kam Hwy that are just two lanes, wind-y, and prone to backup.
You might want to look at maybe making a quick stop at Valley of the Temples while you're heading up that way, and fitting in Kualoa Ranch somewhere in your itinerary. Right across from Kualoa Ranch is a beach park with a great view of Chinaman's Hat, but it's not really a great beach to swim at.
Manoa Falls is easy to get to from downtown Honolulu, and it's a pretty easy hike. Manoa Valley does see a lot of rain and precipitation, so be careful of mud, fallen leaves, and roots.
Snorkeling at Ko Olina does not compare to Hanauma Bay. If you have time for Hanauma Bay, do it!
Sharks Cove will be amazing if the waves are high and in December they likely will be. It's such a powerful sight and I can sit in the sand and watch for hours. Look for beaches the locals like at sunset. Visit Laie Point for some beautiful sunrise views. I could go on a long time about this.
