Here are a few ideas:
Between Sea Life Park and Waimanalo, stop at Makapuu Point for body surfing. Best on the island.
Between Waimanalo and Kaneohe, stop in Kailua and check out Kailua Beach Park. One of the nicest beaches on the island. Great place for a picnic lunch. You can get deli-style lunch stuff at Kalapawai Market, at the far end of Kailua Beach Park.
www.kalapawaimarket.com
Lanikai is the area adjacent to Kailua Beach Park - some incredible multi-million $ homes there. Nice to drive by and daydream over.
http://www.hawaiiweb.com/oahu/beaches/lanikai_beaches.htm
Don't miss the Pali Lookout. Amazing, very windy views of the windward coastline. Take a walk down the Old Pali Road, which is hanging off the cliff below the Lookout. That's the highway islanders used to drive on before the Pali Highway was completed in the 60s. Scary place!
http://www.hawaiiweb.com/oahu/sites_to_see/pali_lookout.htm
In Hauula, stop by Kualoa Ranch. That's where they film "Lost," some of "Jurassic Park," and a bunch of other movies. They have an interesting gift shop - and you might see somebody famous. Or a dinosaur...
http://www.kualoa.com/
Waimea Bay has amazing surf, if you're there in the late fall (like right now.) At other times of year it's a great beach to hang out on.
http://www.hawaiiweb.com/oahu/beaches/waimea_bay_beach_park.htm
In Haleiwa, right before you get to Matsumoto's, stop at the Coffee Gallery for the best coffee drink you've ever had. (Makes Starbuck's taste like CHARbucks!)
http://www.roastmaster.com/
In the same shopping center, check out the Surf Museum. It's run by a retired pro surfer from the 60s. Filled with incredible surfing memorabilia. Donation to get in is like a dollar. Well worth the stop.
http://www.hawaiiweb.com/oahu/sites_to_see/haleiwa_surf_museum.htm
After you've done the usual stuff (Pearl Harbor, Waikiki, Diamond Head, Aloha Tower, Hilo Hatties downtown, Ala Moana Center, Iolani Palace, Bishop Museum, and so forth), ask for directions to Tantalus Drive. It's a FANTASTIC, winding road along the top of the mountains above Waikiki. You'll see views nobody ever sees, including looking DOWN into Diamond Head crater. (As I recall, Punahou Street connects to it. Check a road map to be sure.) This is one Oahu sight everybody should see at least once.
www.hawaiiweb.com/oahu/sites_to_see/tantalus_lookout.htm
Have a great trip!
Dave