Swim the Chesapeake Bay or sail down the Potomac River without getting wet (or leaving your chair) or you can explore the Appalachian Trail. How about seeing Zion Canyon and its surrounding trails?
Terrain 360 uses Virtual tour technology to allow people to explore nature. Students have used their tours to visit places they talk about in their studies, and some of their tours have even highlighted environmental issues like land erosion.
You can see all their tours here, or you can read a nice article about them and how they shoot their tours here.