I think this video is a great way to start limbering up your brain. It looks at the relative sizes of planets and some of the biggest stars that we know about.
The scales shown in the video are truly difficult to wrap your head around and it only really talks of the size of various stars. The sizes of stars are nothing compared to the distances between stars themselves, which are again tiny compared to the distances between galaxies. And then everything is dwarfed by the distances between galaxy superclusters!
Truly mind bending stuff.