Oahu
O'ahu is where most people live in HI, so it's pretty crowded.  That doesn't mean it's not great (I love it), but don't expect to have tons of space to yourself at any particular beach or whatever.

Food
- Side Street Inn -- local fare, pan fried pork chops and fried rice are great.
- Diamond Head Grill -- quick plate lunch, rice + mac salad + something (kal bi is really good).  Get a plantation iced tea.
- South Shore Grill -- fish tacos.
- Foodland (grocery store) -- poke.
- Kimchee II on Waialae -- kimchee soup was my cure-all when I didn't feel well.
There's tons of great food, those are just some highlights for me.

Culture
- Hawaii really is a special place.  You can see stuff like the Polynesian Culture Center, which people like to see different kinds of Polynesian dance at their lu'au.
- For more Hawaii specific, 'Iolani Palace is definitely worth checking out.  This is where Queen Lili'uokalani was imprisoned after the Kingdom of Hawai'i was overthrown by American business and military interests.  It's beautiful and sad and every docent is extremely knowledgeable about those events.
- Germaine's Lu'au is one of the oldest, most popular lu'au on the island.
- Nu'uanu Pali lookout is another historical location.  This was the site of a decisive battle in the unification of the islands...  It's also a beautiful place to look out and see the windward side of the island.  Quickly and easily accessible.  We don't bring pork over the Pali.

Beaches
- Waikiki beaches will be crowded, but some are really nice.  Kaimanas or San Souci is on the zoo-end of Waikiki and is a great place to hang out near the old War Memorial.
- There are nice lagoons out by the Disney resort (west side of the island) that are nice to hang out as well.
- Makapu'u is one of my favorites on the east side, depending on the time of the year.  Can have big waves but is really fun for bodysurfing.
- Kailua Beach and Lanikai are on the windward side and are pretty easy going.  You can rent kayaks at Kailua Beach and paddle out to a small bird sanctuary island.
- Waimea is my absolute favorite beach, on the North Shore, but if you don't arrive early parking is terrible.  Jump off the rock.  Swim.  Then go to Hale'iwa and eat at Cholo's or Banzai Sushi Bar (if they're still there).

Hikes
- Someone mentioned Kualoa Ranch already, it really is beautiful and you've seen a lot of it in movies.  That's sorta the breakpoint between Windward and North Shore.
- Makapu'u lighthouse is a paved trail that ends where you can try to whale watch, take some good photos.
- Diamond Head has a pretty easy going hike (a couple of steep stairs) that leaves you with a view of Kapi'olani Park, the Zoo, and Waikiki in general.
- Koko Head is a steep set of stairs but is another place to get a nice view.
- Don't even try to do Stairway to Heaven, it's not safe and you don't want to be airlifted off the mountain.
- There are other loop trails like Aiea Loop and so forth that are just a couple of miles, but hikes are free and fun.

Night Life
- Most of this is in Waikiki; most of it will underwhelm.  But there are some cool places to have a drink and wind down in the evening.

Shopping
- Ala Moana Mall is a big mall, has some Hawaii specific stuff but honestly shopping isn't great out here.
- There's no need to go to the outlet malls or anything like that, this wouldn't be a great use of time.

Other Stuff:
- There are three major highways on O'ahu, and most people are going the same direction at the same times of day.  If you plan your trip to do the opposite (i.e., stay in Honolulu/Waikiki, drive out of town in the morning and into town in the evening), you'll avoid a lot of the traffic.
- Don't plan an overly stuffed schedule, because things move slower in HI in generally.  Take it easy.