This guys goes into a crazy amount of detail in designing these alternate Earths, and other fertile planets:
http://www.worlddreambank.org/P/PLANETS.HTM
(ignore the fact that when he gets to the "biology" section, these planets always seem to be inhabited by furries.)
A lot of it reminds me of my days playing SimEarth in high school. I also liked to draw imaginary land masses and coastlines and stuff, and loved the made-up maps that come with fantasy novels and console RPGs.
And since I'm working on an MMORPG, i actually got to imagine and draw maps during the early pre-production phase of the game.
Maps are cool.