@Elmo STM - I downloaded your "all themes" box tonight and it is pretty nice. I have done about 6 themes on my box and I am not sure I will finish since I now have access to your version.
The only difference between our boxes so far is I use a trigger area instead of a trigger button and I added a text creator to tell you what theme you are entering.
The one special thing I did on Radiator springs was have the storm sky default when you enter the area. As soon as the Text is dismissed, Chick hicks displays some more text and tells you to hit a button to make the storm go away.
Hit the nearby button and the sky changes to the regular sky.
Do all of the terrain themes have a related race track theme as well? I included one split track, one the intersection and the sewer piece. I may remove the split piece though since it doesn't really show you that much. I tried to make mine as compact as possible to save some space. I did leave a length of track between each theme though.