Not sure where you're going but here's my list of must see places/dives:
On Oahu, the Pearl Harbor/USS Arizona memorial.
Stake out your 2x4' patch of sand on Waikiki beach if you like to people watch. Otherwise there's nicer beaches elsewhere on the island - a couple of good ones on the east end of the island. Personally I found Honolulu a little gritty.
Diving with Justleesa:lol:
If you get to Maui: Dive Molokini Crater - either the front or the backside depending on your experience level. If you stay near Kihei, go with either B&B Scuba or Ed Robinsons - both get high ratings here on SB. Also for golfers, the Wailea Gold or Blue courses are excellent.
Also dive the Cathedrals off Lanai with either of them, or Extended Horizons or Lahaina Divers if you stay farther north near Lahaina. It's about a 45min drive between Kihei and Lahaina so where you're based typically determines who you dive with as all the boats leave really early in the morning to beat the afternoon tradewinds.
There's a Luau in Lahaina that I've heard isn't that bad - not my thing. It's right in downtown Lahaina - also good for shopping/gallery crawling.
We also regularly stop at the Maui Ocean Center(aquarium) in Maalea.
If you want to burn a dive day, the Haleakala sunrise tour is awesome, you can even bike down afterward. There's also helicopter flights that take you to areas in Maui that you can't get to any other way - no roads. But no diving for the prior 24hrs. due to the altitude.
If you make it to the Big Island - Night dive with the Mantas off Kona with Jacks Diving Locker or one of the other operators. And a day trip to Volcanoes Natl' Park - especially good if something's erupting.
And if you make it to Kauai, dive Sheraton Caverns with Seasports. And visit Waimea Canyon - it's been called the Grand Canyon of Hawaii. The Poipu Beach area on the south end of the island is especially nice, it makes the list of top 10 beaches on the Travel Channel regularly. It's one of few places to see Hawaiian Monk Seals occasionally sunning on the beach (from a distance, they're seriously protected)
If you're limited on what other island to visit besides Oahu, Maui would be first on my list. End of the tour...
btw it's MaHAlo...