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.
Step 1: Move your subject off-center
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.
Step 2: Include a foreground subject
Consider the first picture above. It's of Mount Shuksan in Washington State. Beautiful, yes... but a boring picture. There are a thousand others just like it, on postcards and calendars.
Include a foreground subject, and you'll add scale, depth, and interest to your pictures.
I like people shots, so even if I'm viewing an incredible landscape, I try to get people into the frame. You could also select an interesting rock or tree in the foreground. Either way, it makes your picture unique and separates it from all the postcard shots that are just "too perfect."
Step 3: Seek out the action
A still picture is, of course, inanimate... but that doesn't mean your pic can't suggest motion. Be alert for those opportunities to capture action.
It doesn't have to be intense; even subtle, everyday movements add dynamism to your pics.
Step 4: Vary your viewing angle
A ho-hum shot can be elevated to an "oh, that's cool" shot just by choosing an unusual angle. I'd wager that 99% of pictures are taken with the lens at the photographer's standing eyeball level... just because it's comfortable.
Make your pic stand out by sprawling on the ground or finding other suitable positions. This is effective with shots of flowers, too -- everybody's seen flowers from eye level, but not so often from ground level.