Try to spend as much time AWAY from Honolulu as possible. Good for a couple of hours to see the 10 Louis Vuitton stores and an ABC on EVERY block, but too touristy and the beach isn't that great.
Hanuama Bay for snorkeling was cool, but make sure the box jelly fish migration isn't going on, because they will shut down the beach to save people from getting stung. I think this happens 7-10 days after a full moon.
North Shore was BEAUTIFUL for beaches. A bit of a drive, but worth it. Not crowded (except for intervals of 5 minutes when a bus load of Japanese tourists unload, swarm the beach and take pictures for 5 minutes and then leave)
Swim with the dolphins @ Sea Life Park. This was AWESOME! (It's also where Adam Sandler worked in the movie "50 First Dates")
Waterpark for the day?
http://www.hawaiianwaters.com/
You can go buy me another pair (like 31 pairs isn't enough for me!) of Vans shoes at the Premium outlets.
http://www.premiumoutlets.com/outlets/outlet.asp?id=29
You can print off a coupon that gets you a free discount book that you can grab from the info kiosk for additional discounts. (remember to buy a luggage scale to weigh all your luggage, so they don't charge you for going over weight from buying so many goodies!)
There's some nice garden/abortoreum walks on the island as well, but can't remember the name of them...
If you're going to Maui, let me know. I can tell you what to do there too. I'll have an idea about what is good on the Big Island as well, but I won't be leaving for another 3 weeks.