for HHI, my kids that age liked the following: fishing, crab pot at the end of the dock (buy at
DVC $5, get chicken necks, etc. at grocery store), swimming in the ocean with just a mask, making sand castles/forts/burying Dad in sand, pm beach hunts for crabs with flashlights, outdoor pm movies at DVC, riding bikes (DVC has all sizes, reasonable rates), crafts (they decorated cork boards to put WDW pins on last year $15, tye dye), smores by the DVC fire pit. At 11, they loved the DVC party by the pool. At 13, too cool for that.
For Hawaii, if you have a car, maybe geocaching. You could put your own geocaches at places in HHI, if there aren't any. Get GPS and practice in your local area/join geocaching online 1st.
For our Hawaii trip that got canx, I was going to go to the Kualoa Ranch and do the 1/2 or full day tour, including movie sites, snorkel off shore, luau, and drive the island, go to Dole pineapple plantation.