Minecraft wiki entry for Superflat_survival, associated text: Raw cod and raw salmon : can only be eaten raw at this time.
Tropical Fish : emergency food source.
Pufferfish : can be than sold to fisherman villager, or it can be used to breed cats. Don't eat it.
Enchanted book : require anvil to applying its enchantment.
Enchanted Bow : can be used as weapon.
Enchanted Fishing rod : use it for fishing, only if it has mending (your crafted fishing rod would still be used as a backup after you get this)
Saddle : used for riding horse, donkey, or pigs you find. (in Java Edition, you can get saddle earlier from ravager who spawn as of reinforcement)
Name tag : can be used to name mobs to prevent them from despawning.
Nautilus shell : useless as there is no heart of the sea.
Lily pad : can be used only for decoration. You can use that to compost as well.
Bowl : if you have mushrooms and bowl, you can make mushroom stew. Also if you have some flowers, you can make suspicious stew.
Leather : if you don't have iron armor, you can use leather to make at least leather armor. Also you can use it to make item frames for decoration and if you have paper, you can make books. You can also make leather horse armor.
Leather boots : leather boots can be useful if you don't have iron/diamond armor.
Rotten flesh : emergency food source.
String : can be used to craft wool which can be than used to craft beds.
Sticks : it can be useful if you don't have enough wood.
Water bottle : can be used to make another water source.
Bone : can be crafted into the bone meal.
Ink sac : can be used only for dye and book and quill.
Tripwire hook : can be used to make crossbows.