OMG I typed out a whole thing last night (Wed is my Fri so I was finally able to take some time to make suggestions), and the DIS went to system maintenance and it ate my reply!!! Ugh.
So now I start over. More brief this time, but I will expand if you have questions.
Assuming you are staying on O'ahu for a couple of days prior to the cruise:
1) Hike Diamond Head. Super easy, cheap, and your kids can totally do it (paved and not strenuous). Sunrise is awesome, but if that is too early, just try to go early to avoid crowds and heat. Many other easy hikes on the island -- Makapu'u, Manoa, etc.
2) Pearl Harbor is a must. So much history.
3) Polynesian Cultural Center. Visit the villages, go to the luau, then the show. An excellent production. Disney caliber.
4) Eat shave ice at Waiola. Yum. The best on the island IMO.
5) If you want to learn how to surf, go to Canoes in Waikiki. If your kids want a more calm beach, go to Kaimana. Love it so much, it's my son's middle name. Great for little ones.
6) North shore is awesome, but far, and the waves will be on the rise. You may not be able to swim.
Maui:
1) Haleakala is just wow. Nothing like it. I think sunrise is the best, but other people have said sunset is great, too. Make sure you layer (it is freezing at the top). Long drive and takes a long time to get up the mountain.
2) I am not a fan of the Ka'anapali side, but many visitors enjoy it there. Lahaina is a very touristy town where there are places to buy souvenirs and yummy restaurants. I much prefer the south side beaches. Makena is my fave, but the current is for experienced swimmers. Wailea is calmer.
3) Road to Hana is super cool, but your kids will be bored. Super long drive on winding road with 50-something one lane bridges. The pot of gold at the end of that rainbow is beautiful waterfalls and pools. But not sure it's worth it with young kids.
Big Island:
1) You have to go see the volcanoes. There really isn't much else to do on the island, and it's a long drive so you might not have that much time.
2) I love Hapuna beach.
Kaua'i:
1) Waimea Canyon is spectacular. The Grand Canyon of the Pacific. Far. Probably boring for your kids, but I would make mine go anyway.
2) I like the train ride at Kilohana Plantation. Your 8-year-old might think he/she is too old for it, but it will still be fun. You get to see native plants and fruits, and they stop so you can feed the animals.
Kaua'i is my favorite island. So green, lush, picturesque. But super boring. It's our version of "the country". Not much to do except the "touristy things".
Hope this helps.