Happy Anniversary!
I have a few thoughts. Although I'm sure they're in the travel books as well, I've done them and can obviously vouch for them. I didn't do too many touristy things as I was staying with family in Pearl City, but here's what I can recommend:
Pearl Harbor, obviously. Just get there very early in the morning; otherwise you'll waste your whole day.
Hanauma Bay is a must. (
here's a nice page.) It gets crowded (especially later in the day), but the beach is nice and the snorkeling is really fantastic. My fiancee and I swam right up to some gigantic sea turtles - it was breathtaking. Two points of advice on Hanauma Bay: rent your snorkel equipment elsewhere beforehand (like
Snorkel Bob's), and bring food with you. The food prices at Hanauma Bay are freakin' outrageous. I suggest packing sandwiches beforehand.
After Hanauma Bay, which is on the southeast end of Oahu, take a drive up the northeast coast. As you're driving up, you'll see plenty of small areas where you can park and look out from the bluffs onto the water. Plenty of great photo ops here! Keep following the H3 highway north and you'll wind up driving through a mountain, which is just kinda nifty.
Climbing Diamond Head is a nice time, but not a must. LOTS of steps...
I found Waikiki to be friggin' annoying. We walked around the area for a few hours and that was all we needed. The beaches are horribly overcrowded and waves are nowhere to be found. Check your guidebook for better options - the north shore, in the summer, might have some better beaches for you to hang out at. Speaking of the north shore, check out
Pipeline on the North Shore, if the waves are high - we didn't get to go here, and wished we had - we wanted to see some kick-ass surfing!
Food:
If you're up at the North Shore, they say that Matsumoto's has the best shave ice on the island. I had it, it was pretty good.
We also ate up in that area at Haliewa Joe's, which was very good.
The best meal we had was at Alan Wong's, which is amazing pan-Asian cuisine - but be prepared, it's quite pricey.
The other two touristy things we did was go to the Polynesian Cultural Center, which was fun, and we did a luau at Paradise Cove. Again, they're pretty touristy, but if you're into that sort of thing, it's a nice way to spend a day/evening.
That's about all I can recommend, I think. Have a great time!