Select Random Biomes as the world type in the world options menu:
This is the result, looking down at spawn, using the seed shown above (-123775873255737467):
From left to right, clockwise, you see a mega taiga, taiga, jungle, ice plains, plains, and swamp.
Now, here is a larger scale rendering of the world (a bit larger than a level-3 map, 1024 blocks square); the number of biomes in 1.7 means that you’ll still have to explore a bit to find all the biomes
Also, as an optional part of this mod, is a cave/ravine/mineshaft mod which modifies cave generation to get actual cave systems and not random caves all over the place, makes ravines more variable/interesting, and a more uniform distribution of abandoned mineshafts (no decrease in spawn rate close to the origin and a minimum spacing between complexes, with a lower overall chance).
Here is a comparison between normal cave generation (left) and modified (right); note that while cave systems are larger and denser, there is also more empty space between them, with no small caves scattered around (i.e. a minimum size was set):
In addition, caves and ravines were made to not cut through the surface (dirt/grass layer, similar to caves in desert/mesa biomes), except for rare small cave openings, one per cave system (more if there is a compound cave system), note that these caves spiral all the way down to bedrock:
Random Biomes Caves: You MUST install the Random Biomes part for this to work, or the game will crash; the changes also only affect the Random Biomes world type.