For how long are you planning on coming? Do you want to just stay on one island or do you want to island hop? It's very easy to do. What kind of vacation do you want/prefer?
Each island is different and has their own special characteristics, so it really depends on what you want to do, experience.
Oahu, the main island is where "waikiki" is. Personally, if I was to come to Hawaii I wouldn't want to stay in Waikiki for more than a night or 2. It's to touristy and commercial. But it is fun to see, beach is nice, but crowded and should be experienced. Oahu is also the most crowded island. Things to do would be the North Shore, snorkle at Hanauma Bay, hike Diamond Head, Pearl Harbor. The new disney property is opening up this August they say. Looks wonderful! I checked rates the other day and for Sept. they were running $399 a night for a standard room

We like the Turtle Bay resort on the North Shore, but you are kinda issolated out there, so again it depends on what kind of vacation you want.
Maui is really fun. Lots to do, lots of action. It's really pretty. Nice beaches.
Kauai, is quiet and beautiful. My fav. island. It's more low key, beautiful beaches.
Big Island is nice too, there's the volcano still erupting and that's a really fun hike to do to see it, if the flow is safe and accessible. Big Island too is low key. Also our fav.
Then there's Molokai and Niihau....
In all, it's really what you want to do, hang out at the beach and relax, do you want to do a lot of sight seeing? But you can def. experience 2 islands and have a great time. Let me know if you need more info/help/hotel recommendations, etc.
As for time of year, August is hot as is September, those are kinda the hotest months. December/Jan sometimes Feb. are rainy months, but you never know what the weather will bring when living on an Island, it can be pouring in the morning and beautiful an hour later, cloudy and ugly on one side and beautiful on the other. Can't tell you how many times I went over the mountain to town and it was rainy at my house and nice and sunny in town. temps pretty much remain constistent, never really cold, never really that hot. I've only lived on Oahu, lived here about 36 years, but have been to Maui, Kauai and the Big Island. So pick your island(s) and I can help you from there.
Also, as for ocean temps, it's doable all year round. A couple weeks ago we were at the beach, I thought the water was cold, but the kids didn't care. But it is alittle warmer not in this time of year. Personally, I'd see when you can get a good flight rate and go from there.
Happy Anniversary