Well, I cannot seem to get past the subject of replacing a boring sky with one that is more interesting. The thing about skies is that they are often easy to mask and that is a big help when it comes to something that can be as time consuming as creating a good layer mask.
This image took just minutes to mask. I went to channels and created a new channel by creating a calculation using multiply mode on the blue and green channels. This did a nearly perfect isolation of the sky with a little help from a overlay brush. I then created a new lay from a sky shot. I then loaded a mask for the calculated channel and bam! All that was left was to soften one edge on the mountain and it looked good. 5 minutes tops!