Tips that I find useful:
a .capx file is just a .zip file, you can open it with a program like winrar and extract the files.
then you can edit the .caproj file with a text editor and change the <saved-with-version> line near the top to your version number (if the .capx file you downloaded is from a newer vesion of C2 than what you have installed). Obviously, the .capx file can't be using any new features not available in your version - but if you are just checking out a tutorial or someone's example this works great.
maybe everyone but me knew this one:
you can use the same event sheet for multiple layouts. My son made a game with half a dozen levels and was running into the free version event limit. We re-organized it so all the levels used just one event sheet (because player movement and interaction with objects was essentially the same from level to level). This gave him lots of free events to keep developing! (we have since bought licenses).