Sure, you can spend many hours experimenting with f-stops, shutter speeds, filters, post-processing, and HDR... but most of the time these quick-and-easy tips are all it takes to make your outdoor photographs more appealing.
You've all heard this one before... but it's the most effective photography tip out there. Focus on your subject, as usual, but before you click the shutter, pivot your camera just enough to put the subject off-center. For some reason, this adds energy and vitality to your picture.
Tip: make your pivot in a way that suggests movement towards the center of the pic. It gives a kinetic energy to the picture, as if your subject is about to move into that open part of the scene. The 3rd and 4th photos above are examples of this.