I've been 6 or 7 times, to all the islands except for Lanai and Molokai. My ex's parents used to live there in the winter so we went a lot.

Will be easier to give you specific recommendations once you pick the island(s) you're going to visit. The resorts there are incredible but honestly I prefer airbnb's and blending more with the local vibe. The entire coastline in Hawaii is public land and all resorts are required to provide public access so even if youre not staying in a resort you can enjoy any of the beaches.

Everyone shops at Costco because regular grocery stores in HI are very expensive. Renting a car is a must on any of the islands.

Re: island hopping - Its easy but it basically takes up a whole day, so keep that in mind. The flights are only like 30 mins but you still have to check out of your place, drive to the airport, return the rental car, go through security, fly, rent another car, maybe get groceries, drive to your new place, settle in. If you're going for 10 days or 2 weeks I think doing 2 islands would be perfect. I've never been to Molokai and while I think it would be interesting to see I'm not sure I would "waste" the time going there vs exploring more of the other islands. I kind of figure we should just let the locals at least have one of their islands not scourged by tourism and haoles.

I recommend reading up on the history of Hawaii before you go. It has an interesting, rich culture and complicated past. You will appreciate your trip more and hopefully be less of a dumb tourist because of it.

As a general rule respect the wildlife, nature, and locals. Don't get close to seals or turtles. Some locals don't play, but if you arent a dickhead they are very warm and nice people. Shaka!

My island ranking:

1. Kauai
In my opinion the most beautiful and best island. Especially the north shore. There are no big resort areas there, and there is more of a local feel. There can be frequent rain in between the sun but you also get incredibly lush rain forest scenery because of that. The southern side of the island is usually sunnier and drier so simply driving a little bit to another part of the island can get you different weather. When I think about going to Hawaii, I think of Kauai first and maybe visiting the other islands in addition.

2. Big Island (Hawaii)
The most diverse island in terms of the land - some extremely wet rain forest, some arid desert like lava fields, some rolling green hills. Although the other islands have more great beaches, I think the BI is a must visit at least once. Volcanoes National Park is amazing and the volcanic activity on the island just gives it a cool energy and good insight on how the islands were created. I love hapuna beach. I don't love Kona - its a cruise ship port and therefore has a lot of tacky tourist trap stuff. I think BI is supposed to be the most affordable.

3. Maui
Kind of a blend of Kauai and BI. Lots of gorgeous beaches, stunning road to Hana (recommend staying overnight in Hana), Haleakala volcano. There are two major resort areas - Kaanapali and Wailea. I think Maui is supposed to be the most expensive.

4. Oahu
Honolulu is a big city. Waikiki is saccharine tourist yuck. However, the north shore is great. It has some of the most famous surf spots in the world, and that culture really dominates the north shore along with a really strong local vibe. This is the most populated island so it is interesting to see more typical local life here. Admittedly I've only been to Oahu once, but I wouldn't mind returning to the north shore.