I was feeling a bit stumped for the larger environment for my local D&D campaign. I decided someone must have created a virtual planet generator. Ask, and Google shall provide!

http://www.eldritch.org/erskin/roleplaying/planet.php

I liked this one:
planet -s .1996062 -w 800 -h 600 -pm -C -c -m 1 -i -0.015 -V 0.13 -v 0.9 -N 256 -S 0 -l 0 -L 0 -g 0 -G 0 | pnmtopng >Planet.1996062.png