A lot depends on taste. I like to fiddle with photos so here goes:
* more contrast
* vignette
* change the green of the grass a little bit
* mist on the grass
* little bloom
* little dirt
* sharpening
These are all basic photoshop tasks. Try it yourself and see what happens.
This guy has many good tips: http://www.alexhogrefe.com/
Hope it helped...