You might want to crop out the out-of-focus grass in the foreground.
I'd also add a bit of fill light in the ones with obviously dark shadows on the subject & dial in some local contrast enhancement. The knob in Lightroom that does this is called "Clarity", not sure what it's called in Aperture.
You can go even further with tone curves, but that's something that takes a long time time to learn how to use effectively...