Here is a fun little practice assignment:
The photo selected is to appear on a pamphlet for a lakeside children's camp. Obviously we need to clean it up to make this look like a good place to bring the kids.
Here's my workflow.
1) Adjusted the lighting using the curves layer.
2) Adjusted color using a separate channel mixer layer for each channel.
3) Then used a combination of clone stamp, spot healing brush and patch tool on several different layers to clean up the image.
4) Flatten image and applied unsharp mask.