Here's a technique that is worth applying at some level to pretty much every image. You probably won't find many images which aren't improved by its conservative application. Generally it should be used subtly rather than bluntly to generate a little extra "punch". You should do this before mapping down from 16 to 8 bits (curse you photoshop!).
What you do is apply the unsharp mask. But instead of using a small radius so as to sharpen up edges and give that horrible "over sharpened computer image effect", you set the radius to be very large. For instance, 80 pixels. Adjust the amount to decide how much of the effect you want, and switch preview on and off while changing parameters. You should play with the threshold - increasing it will cause the effect to be limited to the higher contrasting areas in your picture. In particular, its easy to get it to leave the blue water and water surface alone in your wide angle shots. For macro subjects you can better separate them from your negative space.
At this scale, the unsharp mask doesn't act like a sharpening filter - it acts more as a smart selective contrast enhancer. You can't achieve the same effect using curves or other global contrast manipulation tools. It will increase the contrast between the large scale elements of your photo while leaving the fine details alone and not magnifying noise.
I view it as more of a tweaking technique for images that are already pretty good, but I'm sure it is also helpful for images in need of radical
repairs, and if you want to get creative you can produce some interesting unnatural effects.
