The Spore creatures, likewise, used Metaballs, a geometry creation algorithm that could create smooth tubes along paths in 3D space. Other algorithms for filling spaces and paths are Voronoi patterns, Perlin/Simplex noise, triangulation algorithms, 3D extrusion or rotation, and the Diamond-square algorithm for fractal terrain. These algorithms are particularly well-suited for interactive generators, because the user can provide the input parameters for the generator.
Because Google is like a jetcar with a 20-speed manual gearbox, first gear is plenty for most people. However, if you want the other 19 gears, here are some ideas to get you fired up (just make sure you’re pointing in the right direction first)…