What are at least two descriptions or items that show the island may not be totally idyllic to Ralph.

I think that as the story progresses Ralph begins to notice that the island is not the fantasy playground that he thought it was. The earliest sign is the scar down the mountain that Golding mentions many times. It is in fact from the plane crash and is symbolic of the boys themselves as antagonists to this perfect paradise. The "creepers" in the jungle also provide an ominous sense to Ralph. Then, talk of the "snake thing" and "beasty" from the boys' imaginations casts another ominous shadow over the island.