While sacred sites can be found in almost any location, there are certain features that often accompany those sites that are found in natural locations. They can often be found within naturally occuring boundaries, such as rivers and streams or game trails carved into the earth by elk and deer. Sites can also been indicated by other features, such as naturally occuring obelisks and circles of mushrooms, trees, or stones. But sacred sites most commonly occur at transition points - places where one natural feature or event changes into another.
While there are numerous transition points, four obvious sites come to mind: earth to sky, earth to water, without to within, and day to night (or night to day.) From a mystical perspective of reality, any transition is considered magickal on its own. Doing ritual work at any of these points draws upon that preexisting pool of transitional energy to change our own reality from one state to another.
Earth to Sky
This is one of the most traditional transition points to host a sacred site. For tens of thousands of years, mankind has been ascending to the heights in order to get closer to spirit and there are ancient ruins around the world attesting to the spiritual draw of such places.

Earth touches sky in the Mt. Jefferson Wilderness Area.
Examples of this type of transition point include mountains (both their slopes and their peaks), hilltops, ledges and cliffs, and other locations that look out upon a lower elevation.
Earth touches water on the northern Washington coast.
Examples of this type of transition point include coastlines along oceans and seas, the banks of rivers, lakes, and streams, and to a lesser extent, wells and sinkholes.
A sea cave leads us from without to within.
Examples of this type of transition point include not only caves, caverns, and lava tubes, but to a lesser extent canyons and forests.
Watching day turn to night as the sun sinks into the Pacific Ocean.
Examples of this type of transition point include any area with either an open line of sight to see the rising or setting sun or a sight-line where this event is distinctly obscured (such as the sun rising over a point on a standing stone.)