Heh, heh.

Well, looking at all the images so far, the main impression I get is that this is all happening on a cloudy, dreary day. Since they're going on a picnic, I'm assuming that it is not a dreary day. My suggestion would be to add a sunlight to the lighting setup. Place it wherever the HDRI has the sun coming from and adjust. I can see the highlights, but there ought to be more on a sunny day.

Of course, I'm only assuming this is a sunny day. :)

I find I often have to add a sunlight, or some other light to 'help out' the HDRI. I've tried cranking up the HDR intensity, but that never gives me the results I'm looking for.

That's all. Your scenes are always much larger than I usually ever attempt. I usually try to watch when you start one, because there's always something new to learn.