Hamlet is being sarcastic in regards to his new relationship with Claudius. Two months ago, Claudius was his uncle, now he is Hamlet's step-father. That is kind of mind-boggling and gross to us, and it undoubtedly is to Hamlet as well. His first line, an aside, is "a little more than kin and less than kind." He is specifically talking about this change in their relationship. When he says he is "too much in the sun" of course he is punning sun/son. He is saying he is too much a son now that Claudius has married his mother so soon after the death of his father. In several lines in this scene, Claudius calls Hamlet his son, which only serves to highlight Hamlet's attitude about all of this. Throughout the play, Hamlet feels dirtied by the corruption of Denmark -- a theme that starts with his first lines.