Here is my quick breakdown of the various islands:
Big Island: BI has a little bit of everything and lot of some things. An active volcano is pretty cool, as well as lush rain forest, near desert conditions, lots of historic lava flows, hiking, nature, and good snorkeling. BI also has beautiful beaches, but they aren't nearly as accessible as other islands. For example, Makalawena is one of my favorite beaches in Hawaii but you have to walk about 15 minutes through a lava flow. So you probably won't be bringing a cooler. And in general you'll be doing a lot of driving to see things. Costco is in Kono, where you likely will be staying.
Oahu: IMO, Oahu is the most scenic--if you could just move all the people off the island. But still has great beaches and nature, tourist destinations and attractions like Pearl Harbor, and a thriving food and bar scene which the other islands lack (and I'd argue, most other American cities lack). There are several Costcos on Oahu.
Kauai: Kauai is all about nature. It has got some nice beaches and snorkeling, although the latter can be hit or miss due to all the rain. But the main thing is the lush, wide open scenery makes it feel like a movie set. Kauai has a very laid back island feel, with a lot of ma and pa operations. It kind of feels more like Hawaii than the other islands, if that make sense. Northern beaches can be rough in the winter. Forget about night life. The whole island is in bed at 9:00 pm. Costco is in Lihue by the airport.
Maui: Maui checks all the boxes. It has everything, especially plenty of great, accessible beaches, but lots of great (and unique) nature too. Downside is everyone knows it checks all the boxes, so it is a popular destination. There are two main areas where tourists stay: South Maui (Kihei) and West Maui (Lahaina). It is hotly debated which is better, but I prefer West Maui. Lahaina has a good food and arts scene. Kihei not so much. But Kihei is closer to upcountry, Haleakala, etc. A big plus for Maui is whale watching in the winter. You can see whales from other islands, but not like Maui. On Maui, most bars stay open all the way until 10:00. Costco is in Kahului by the airport.
General tips:
Buy some roll up bamboo mats at the ABC store. They are great for the beach.
I like microfiber towels (like they sell at REI for camping) for beach because they don't collect sand like beach towels.
I strongly recommend getting rash shirts for everyone. You'll use way less sunscreen, and the tropical sun is no joke.
Speaking of sunscreen, Hawaii requires reef safe mineral sunscreen. There are no sunscreen police, but that's the right stuff to use.
Most resorts and larger condo complexes have give and take boxes where people can leave their beach chairs, mats, beach boys, etc. for the next guest (less so since COVID). It is totally hit or miss, but you can score big. So check there before you buy anything. Pro tip: You don't actually have to stay at the complex to raid the box. Just ask the front desk where the box is.
Similarly, there are various Facebook groups for people checking of their condos and leaving food behind.
Sign up for the snorkel report. You can rent snorkel gear (along with beach chairs, boogie boards, and such if you like) at various places like Boss Frogs or Snorkel Bob's. You can drop off at any location.
If so inclined, Costco is a great place for Aloha shirts. They are $20, nice fabric, pocket pattern matches the shirt, etc.
Pack everything you think you will need, then dump out half of it. You won't need it. Some t-shirts, flips, and some shorts is all you need.