What do the beach, beast, huts, forest, darkness, island, and Castle Rock represent in Golding's Lord of the Flies?
Wow. This is quite a list of items, each of which play a significant role in Lord of the Flies by William Golding. Your use of the word "represent" implies that each of these elements in the novel is somehow symbolic, but most of them probably do not rise quite to that level of importance. The beast is an obvious exception, and if I were adding symbolic items, I would certainly add the conch, fire and Piggy's glasses. That does not mean, however, that the items on your list are not significant to the story.
The beach is the place where everything begins. This is the place where the boys meet, and it is a place of relative safety. The most awful things that happen on this island happen on the mountains; the beach is the last safe place until Jack and some of his savages come to steal Piggy's glasses. When Ralph blows the conch, the boys know to meet on the beach; by the end, he is afraid to blow it because he intuitively knows that no one will come. Ultimately, though, the beach is a place of rescue. Not only is there nothing much to burn there, but it is the place where the naval commander finds and rescues the survivors.
Early in the novel, Castle Rock is the place where Simon, Ralph, and Jack enjoy pushing rocks down the mountain. Later, of course, this is the site of the new tribe--Jack's tribe of savages. What begins as a castle, a place of imagination and fun, later becomes the home base of savagery on this island.
The beast is a consistent element in this novel, though of course we really never meet any real creature that might be considered an actual beast (that's how we know it's a symbol rather than just an object). Though we hear about beasts from the littluns who are waking up and seeing tendrils hanging from the tree that seem to be snakes and from Samneric who run down the mountain in terror and describe an outrageous creature, we know these are fears and not realities.
What we learn about the beast on this island is that it is the boys' human nature that is the beast. In chapter 5, Simon hesitantly says that maybe it is them, but the boys mock him for his comment. In chapter 8, Simon learns the truth from the Lord of the Flies who tells him:
There isn’t anyone to help you. Only me. And I’m the Beast.... Fancy thinking the Beast was something you could hunt and kill! ...You knew, didn’t you? I’m part of you? Close, close, close! I’m the reason why it’s no go? Why things are the way they are?
The beast is what is inside them which causes them to turn on one another, their human nature without any boundaries or constraints.
The huts are a place of safety, but they are inexpertly made. Ralph and Simon try, but the other boys aren't very interested. The huts are indicative of the false sense of safety and security the boys have on this island--mostly because the worst dangers are found within the shelters rather than without.
The forest is where we see the "scar" left by the airplane when it crashed. It is a foreshadowing of what is to come, and it doesn't take long for that foreboding to become a reality.
The darkness, of course, is the worst time for the young boys. Though they are used to being away from home (they are boarding school boys, after all), they consistently have nightmares. Also during the night we see the only signs of what is happening off the island.
Finally, the island is a microcosm of what is happening in the outside world. Just as there is a war of sorts being waged on the island, the world is also at war.