Just recently my two DDs (who are both out of college) have asked about doing a family trip together. Last big trip we did was to Iceland when they graduated. One of them suggested a “warm place” and the other suggested a cruise. We’ve done a Caribbean cruise out of Houston. Husband is thinking somewhere in the US. BTW, we live in Chicago, DD1 lives in Houston, DD2 lives in Seattle. I suggested Alaskan cruise but of course it’s not a “warm” place. I thought also Kauai since we haven’t been there although we’ve been to Oahu, Maui and Big Island. Any suggestions? I don’t think we’re limited to time of year.
My family has gone to Kauai several times, often meeting other families there. It is pretty developed, but there are so many activities it’s always a great time. It’s easy to get around if you rent a car and you can go hiking, kayaking, zip lining, horseback riding. It’s a great choice. We did have a wonderful time on an Alaskan glacier cruise (Anchorage to Vancouver) last summer. We were on Norwegian Cruise Line and they offered some wonderful excursions and the ship was well-run. There were activities for all ages and many families. It was cold though, and there were times we wished we were sitting on a warm beach. Have you thought about the Florida Keys if you want to stay in the U.S.? I would also look into Costa Rica if you are willing to travel a bit further. I have not been, it’s on my list of possibilities, and I know many people who have enjoyed it.
Want to stay in US? How long a trip?
What do you enjoy doing with your travel time? Iceland is very outdoorsy. Did that appeal to all of you? Do you prefer to be active or sit on a beach? Cities or nature? Cultural stuff?
I vote for going out of the country as you can always do trips in the USA when you age. 
Would you be interested in a rustic adventure featuring mostly-deserted beaches, great snorkeling, getting around via golf cart or Jeep, “dangerous” hikes on “forbidden” trails, and reasonable prices?
If so, consider Culebra. My wife and I loved it.
Ah, and lest I forget, even the most famous beach – Flamenco – has something quirky about it: an old tank resides there.
It is so cool, a tiny nearby island is named after it: Culebrita. (Culebrita is a day trip must – an unbelievably calm beach, rocky baths, goats…)
How about an all inclusive someplace in the Atlantic Ocean? Warm, and there would be things for everyone to do.
I LOVE Bermuda, either as a cruise destination or staying at one of the resorts.
We’ve done London, NYC and Washington, DC with adult kids.Not at all like what you are considering, but I thought I’d throw in a different idea. Probably won’t work unless everyone loves museums and galleries as in our family. We didn’t try to have one itinerary for all to follow as each person had their own priorities for sightseeing. Sometimes excursions were shared with others, sometimes some went solo. We always had a nice evening meal and simple-start-the-day breakfast together.
How about The Galapagos? That’s one of the places on my list, along with Panama Canal cruise.
Just because I have to add in: How about Disney World?
You said the US, so I’m thinking one of the national parks. Yosemite, Grand Canyon, Bryce, …
Sanibel?
What about the Florida Keys?
Sedona, Arizona
Do something EXOTIC… something your kids aren’t likely to do on their own in many years.
Patagonia? Zanzibar? The Orient Express from London to Venice?
How about San Diego? It’s gorgeous there weather wise. Lots to do. Short drive to the mountains ( go to Julian and have apple pie). Nice drive to the desert. Beautiful beaches. Balboa Park. Good food scene. And decently easy to,get to flying wise for everyone.
Grand Circle? Grand Canyon, Bryce, Zion, Page/Lake Powell, Arches, Moab, Monument Valley. You could stay in one place (VRBO) and make day trips or just go from place to place. Fly in to SLC/LAS/PHX, any of those would get you close to the start of that journey.
Our favorite family vacations have been Alaska (cruise plus a week touring on our own including Denali) and a week out west hiking Grand Canyon, Bryce and Zion. Highlight of that trip was a 3 night rafting trip through the Grand Canyon – sleeping on a cot under the stars at the bottom of the canyon.
I would recommend that wherever you go, go during low season. Since you are not constrained with school schedules, get the most bang for your buck.
Kauai offers all kinds of things to do for every generation. However, there is really no nightlife if the Ds want to go out at night together
I second San Diego!
We just got back from New Orleans (non Mardi Gras) and we had a blast with our adult kids.