I expect you will get some replies that are exact opposites of each other, but that’s good! You’ll get pros and cons of stuff that way. The thing one person hated about something might be a thing you love....
iMO the recommended length of stay is “as long as you can afford!!” You could easily stay a month and not do anything twice—the islands are small but there’s so much to see. Same answer regarding how many islands—as many as you can!
Realistically, you could look at all the things to do on all the islands and then decide which activities interest you more, and let that drive the decision. Like if Pearl Harbor is a must, you have to do Oahu. Want to see live lava? Only on the Big Island? Black sand—BI or Maui. Those kinds of things. Honestly if island hopping turns out to not be an option, you can have a very interesting trip only on Oahu. (I’d say the same about Maui or BI too, if you’d already seen Pearl Harbor.)
Now the must-do’s are where it will go all over the map! But with all adults, the Dole plantation would NOT be on my list. It’s important to know if these are active, adventurous adults or more onlooker types.
I’m assuming active with my must-do list. I say Hanauma Bay is a must on Oahu—some say too touristy but it’s very easy snorkeling in a huge (uncrowded) area. We loved it. I also think climbing up Diamond Head is a must, for the views, including down into the crater. Some say the Tantalus lookout is comparable views without the climb if that doesn’t appeal or make the cut. It was more strenuous than I expected (since it seems like everyone has done it) but plenty doable. I’d put Kualoa Ranch on my list for “next time” on Oahu; we just couldn’t do it all. Waikiki Beach was one and done for me; I’m not into the urban scene when there are beaches to be had! The free hula show was good though, if you want to see hula without a luau. Although I’ve always heard great things about the Hale Koa luau. And of course, you have to stay at or at least visit Aulani!!
I’m currently planning to go back to Maui for everything we missed there, but I’m making more time for the black beaches this time (especially Wai’anapanapa—possibly my favorite spot on the planet). Also the Pipiwai Trail through the bamboo forest. Both of these things are near Hana, and we drove the road (a must-do ONCE) before but want to revisit the faves. Haleakala is a must, although if you can’t handle sunrise, sunset or broad daylight are cool too; it’s worth seeing any time of day. We also enjoyed driving though upcountry to see the farms and vineyards, and we plan to go horseback riding this time. My biggest must for this next trip is to take full advantage of all the great free snorkeling right from the beaches. Molokini was #1 on my list, but the reviews (including from a friend who goes yearly) have really declined in the last few years, and I’ve decided to nix it. A must for me somewhere in the trip is a luau. Thought we’d do a sunset sail for DD’s 21st bday, but she’d rather do another luau!
On Kauai, Waimea Canyon is a must. It’s an easy drive but make sure you go all the way to the top; the road dead ends into the parking lot, so you can’t miss it. The whole island looks like Jurassic Park, so just driving around is awesome. We saw monk seals at Poipu Beach, so that was awesome. Another must is seeing the Naapali coast, and you can do it by air or boat. You can also hike it, but I think it’s about 10 miles to really see it and we weren’t up for that. We saw it from a cruise ship and it was amazing; a catamaran would be awesome too.
And on the Big Island, my main agenda was LAVA! Actual molten rock! Enough said. The lava tubes are fun too and I’d say a must IF you’re going to Kilauea. But I wouldn’t go to the BI just for the tube, KWIM? I also had the green sand beach on my must-do list....and was so bedazzled by the lava that I forgot all about it!! LOL. Kona Brewery was a must for me because I was on the island but again—I wouldn’t go just for that. Rainbow Falls was also pretty but not life-changing. (My kids were actually underwhelmed by the waterfalls in general—“we’ve got waterfalls that big at home...” Go figure.)We only spent two days on the BI, so I’m sure I missed several musts.
And IMO, no, I’ve never used a TA and don’t see the need. Between the “Revealed” guidebooks and the internet, I did my own research and made my own plans, including air, hotels, a cruise, and rental cars on every island.
So WOW that’s prob way more than you wanted to hear!! But just get familiar with what’s actually there before you decide where to go. And this forum is a great place for info. Happy planning!!
But that might just be worth it. With PH, DH and HB all being a priority, and best done early I may just have to do that. I wouldn't mike doing DH very early and getting tickets to PH 60 days out for an afternoon time slot, which would give us time to go back to Aulani and shower and clean up well.
