Over in Tomorrowland, the PeopleMover would offer an overview of the area:
Space Mountain joined the Magic Kingdom's line-up in 1975:
The park's second "mountain", Big Thunder Mountain, had originally been envisioned as part of Thunder Mesa, which was to be the first expansion to the Magic Kingdom. However, this was parked in favor of building a version of Pirates of the Caribbean, and it was later revisited as a standalone attraction. Here's an early concept sketch, followed by a more detailed treatment:
Another mountain, Splash Mountain, opened in 1992.
The New Fantasyland expansion is almost complete, with the Seven Dwarfs Mine Train due to open in spring 2014. Here's some final concept art showing off how Disney envisioned its latest update to the Magic Kingdom:
Comments
Pretty sure that Frontierland picture with the Haunted Mansion in the background is Disneyland Paris, not the Magic Kingdom in Walt Disney World.