Tablets will take the longest to complete, You will only see coral on the beach or reef. Shells are only around waterfall and what I call "The Love Shack" Feather can turn up anywhere. If you use your number keys on the side of your keyboard you can roam the island quickly which makes searching for collectables easier. You are going to have to be patient since you already finished the puzzles. Or you can start a new tribe(different faction maybe) and always be on the lookout for collectables.
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Love Harry Potter, and Virtual Villager.
Love my Sims too, but they are expensive.