Hmmm...the difference in softshadow-diameter is noticable but not as dramatic as I hoped it would be. Maybe crazy values are necessary, but I think I'd keep it like this. The benefit isn't big enough compared to the increase in rendertime, probably.
So about ideas for trees/bushes and clouds etc: I think it would be cool if the background hills had patchy pieces of tree-populations. In the foreground you could use some bushes, but I won't touch it too much. It's nice and "quiet" and I like that, just my taste.
For the atmosphere: your whole image is now "filled". You have a foreground with grass and sand, then water with canoes and then background hills. Also the atmosphere is filled with clouds which makes the image "busy".
I'd either completely remove all the clouds or make some very thin and small patches of streaky clouds.
Hope you'll like some of these suggestions

Martin