Avoid bright flowered beach towels due to bees. Last trip came out of the ocean after snorkeling and started to dry off and got a bee sting on my eye lid. Not a fun day. In recent trips I am packing my oldest beach towels and will just donate them at the timeshare at the end of our trip.
Pack Keens/water shoes that fit well and are comfortable. Avoid thin flip flops- some of those occasional big nasty thorns and other unseen hazards, sharp lava rocks, coral, are in and out of the water and on the trails to many beaches in Hawaii.
Have a secure waterproof place to attach your keys/id to yourself if everyone is planing to go in the water. I found that a neoprene camera bag worked well to ziploc keys, ID ,CC in. I also have lanyard pouch for my Iphone if I am bringing it.
Buying boogie boards at Costco if you are going more than a week and plan to use them will be worth it Check for loaner first. Don't buy cheap on at the convenience store.
Favs- Love Lampert's Ice cream-Kauai and Maui, Hula Dog in Oahu, Maui-Da Kitchen-Kalua Plate lunch (luau food, ono$), Geste Shrimp Truck, Ualani's shave ice, Purple Taro bread rolls-from Big Island factory in loaves, can buy at Kahalui Costco