creating planets in blender

I want to create planets in blender. What is the best way to do this? Should I just make the spheres and then manually edit them there? Is it possible to walk on the surface of this sphere once I make it? Should I just make a textured planet and then make a flat 2-d level that represents the surface and when you get so close it turns into the 2-d map? Are there faster ways to do it? I've been doing alot of research and the procedural stuff I've found haven't been very good or helpful and I don't even know if thats what I really need. Hope some one can help!

There are a few questions in there. Firstly to learn Blender you should talk to the Blender community:

There are a number of ways to make a planet you can walk on. The easiest would be to have a layer of clouds and when the player hits that layer, unload the sphere and load a flat terrain.