Stamp using custom brushes and content-aware fill
One tool that I use often is the Stamp Tool using a custom brush. The most common form of this would be stamping in vegetation using a scatter brush that emulates the vegetation. It can also be used for clouds and trees in the far background. I use this technique during the last phase of fine-tuning and when adding any tiny elements such as the trees on the very top of the large mountain in the background. Content-aware Fill is also very useful (Shift+backspace, set to Content-aware) to quickly patch unwanted elements such as wires, houses and so on.