Already great information here, just throwing in my two cents!
1) What is the recommended length of stay in Hawaii for a solid first visit vacation? We will be visiting Disneyland for a couple of days on one end!
Our honeymoon was also our first visit and we were there for 16 day across three islands. If/when we are able to visit again, we will likely do 10 days and maybe two islands. As PPs have mentioned, it takes a good 2-3 days to acclimate to the time change. Hawaii does not observe daylight savings, so they were 6 hours behind us coming from NY when we visited in September. For the first 2-3 days after arriving, we were ready for bed by 8-8:30pm and up by 4-4:30am. Takes some time to shake off the jet lag and feel back to yourself after that kind of travel and time change.
2) What islands and how many days at each would you recommend?
This is really going to vary depending on people's preferences. Many here say you need a good week to truly experience everything there is to see and do on any particular island. I don't disagree, but we will never have that kind of time to dedicate, so we had to be selective about how we approached each island.
We first spent 3 nights in Waikiki (Oahu) which is when we did Pearl Harbor and Diamond Head - both of these are best done early in the morning, and it worked out well for us adjusting to the time change in the beginning. We also did not have a rental car during this time due to the traffic/parking costs and used organized tours for these two spots, as parking can be a challenge at both. From there we flew to Maui for three nights, staying at the Westin in Kaanapali. The resort was gorgeous so I baked in resort time, and we also visited Lahaina and did a luau here as two of the highest rated luaus in Hawaii are in Maui. We would have had plenty of time for the Road to Hana or Haleakala, but opted not to do it. From there, we flew to Kauai for three nights and stayed at he Marriott in Lihue. We did a sunset dinner cruise around the Na Pali Coast and a helicopter tour of the Na Pali Coast and Waimea Canyon. We could have made time for hiking at Waimea Canyon, but DH and I are not much for hiking and I wanted to keep our itinerary less than packed to allow some relaxation and flexibility. Finally, we flew back to Oahu to spend 6 nights at Aulani. We did get a rental car for this portion and planned our days doing every other as a resort day and a touring day. Visited the North Shore, Kualoa Ranch, and Kailua during this time.
In our experience, everything worked out perfectly and exactly as I wanted it to. I didn't feel we had too much or too little time at any spot we visited, and would definitely plan it the same way in the future. Now that we have experienced what we wanted to, we would likely spend the majority of our time on Oahu since it offers the most for us to do - it has a little bit of everything from beaches to culture to history to hiking to shopping. The other islands all have bits and pieces, but Oahu has it all. Kauai would be our second choice to return to because it had the most natural beauty and felt the most exotic to us.
3) What are some "must-do's" apart from Pearl Harbor and the pineapple plantation? All adults in my traveling party!
I would by no means consider the Dole Plantation to be a must do, especially with adults only in your party. As mentioned, its a kitschy tourist trap. We did visit and are glad we did, but wouldn't return. Definitely something I would only do if time permitted and you don't have to scrap anything else in favor of it.
For adults, the North Shore, Kailua, and Waikiki would be my must do's. We loved the relaxed pace and scenery in the North Shore and Kailua, and the beaches in Kailua were the most gorgeous we saw in all of Hawaii. Swimming was also great there. Waikiki is a touristy area, but a ton of fun for people watching, having a nice dinner, grabbing drinks, shopping. It has a completely different vibe than any other place we visited in Hawaii, and its about the only area you'll find some semblance of night life. We found that most of Hawaii is pretty quiet in the evenings, and acclimated to going to bed early and waking up early because of that.
4) Would I be better off using a travel agent to weed through all of this? If so, do you have one you recommend?
Even though I did a ton of research and picked out most hotels and activities on my own, we booked everything through a local agent that works for national agency. We wanted the peace of mind that they had a 24 hour help line, we wanted excellent travel insurance since it was quite an expensive vacation (they had options), and we wanted someone with knowledge of Hawaii to confirm my plans made sense and make sure I wasn't leaving out anything that should be a must do. I believe TA's also have the ability to pair flight segments together for you and can give you plane information, which was important to me. I really wanted a flight that arrived somewhat early on the way in and left late on the way out, and I wanted a certain seat configuration and entertainment options. I was willing to pay more for these amenities because I wanted us to be comfortable, and our TA was able to help with that.
Dreams Unlimited runs these boards and can certainly help with your trip planning. There are agents who specialize in Aulani bookings, but also have a great familiarity with Hawaii. Tracey Heinrichs is one - she's booked a cruise for me in the past and is very quick and easy to work with. She's also been on Dreams' Aulani podcast and I took some of her advice when we traveled there.
On a related note, I assume you're planning to stay at least part of the time at Aulani since you're posting on the Aulani forum? Being that your party is adults only, just wanted to clarify that. DH and I enjoyed it as Disney fanatics, but we were on our honeymoon and its still a very much family oriented resort with kids abound. Evening options are relegated to drinks at Olelo Room, sometimes with live music, and there's not much else to do in the immediate vicinity of Ko Olina. I do think its worth at least a visit for dinner or even short stay for the Disney fanatic in Hawaii, but just throwing this out there so you choose your lodging accordingly