Worlds within worlds within worlds...
Inspired by the movies Cube/Hypercube/Cube Zero, Tesseracts, Dr. Who, fractals, and the description of the Abyss, this is less of a campaign or adventure idea and more of a method for randomizing a large/infinite space.
Buy a Classic 3x3 Rubic's Cube, a 4x4, or if you're adventurous (read: EVIL), a Rubic's Professor (the one with 25 blocks per side) (
http://www.rubiks.com/). Number each block face, so each now has a color and a number, such as Red 15, or Yellow 24. On the Professor, there are 150 spaces.
Randomize the Cube- your party starts in a room/space represented by a square of your choosing, say, Blue 13, which should be face up (parallel to the surface of the table). As the DM, you design a set of encounters in the space designated Blue 13- its size, occupants, etc, like you would any room of a typical dungeon, or a country or even a plane. There are 6 exits from that space, 1 to each side of the square (the cardinal points in 2 dimensions) plus one each "down" and "up," represented by the other sides of the Cube. When the party completes their objectives in Blue 13 and tries to leave that area, manipulate the square on the cube any
odd number of times (you determine how many based on your campaign design). They must leave via one of the 6 exits, which will take them to another space, which will have a color & number which will cross-reference with your notes. Leaving via the 4 cardinal exits is easy- you go to the square in that direction. "Up" and "Down" may be randomized by any method you use. I plan on using 2d4- the first one determines in which direction I rotate the cube, the second, the cardinal direction through which the party exits. Double 1's means the party goes to the corresponding square on the opposite (face down) side of the Cube, and Double 4's means the party picks a square on the face-up side of the Cube.
The campaign ends (if you so choose) when the Cube is manipulated to having all 6 sides with single colors.
Obviously, with only 150 squares and billions of face combinations, you'll probably need to do some restocking...but that's the easy part.