Well, I don't think a plane would be too good of an idea with animation in the scene. To make a plane you can simply make a cube, convert it to mesh, then delete all the points on one half of the cube, and you're left with a flat plane. The sky is a good idea though. What you could do is, create a dome, then apply a cubemap to the dome, and put it around the scene however large you want it to be. To make animation easier after you put the dome in the scene, select the dome and press 'h' which hides the object. Then when you're done animating and want to render, just press 'shift + h' or go 'edit>show all'
