Best Chicken Katsu at rainbowRainbow drive-in just south of there. Great local place for plate lunch. Buy a t-shirt or two.
I prefer the Leeward drive-in in Waipahu myself. (you will only find local there though)
If youre bringing back chocolates, coffee or treats--buy at Costco and not ABC or Walgreens/CVS
If your in to hiking--climb Kokohead. Get to the parking lot about 3:30am and hike up the 1000+ steps (think its closer to 1200). Bring flashlight and some water/food. Can be difficult (ive climbed it in slippas) in some places but worth the sunrise.
Always recommend Kualoa Ranch. You can do ATV tour, Zip lines etc. Very cool. Lots of movie history there. 50 First Dates, King Kong, Jurassic Park just to name a few.
Pearl and Punchbowl in same day is easy to do. Then can hit Pali lookout (windy place).
If you want good local food--eat where the locals eat and not at a place you will recognize the name too. Just ask a construction worker, a gas station attendant, delivery driver--they will tell you.
Be good to the people and their land--and have a good time.