Playing with Bash scripts is fun, if you have any nerd blood in you.
There are piles and piles of open source applications of various types that you can download and try out...you might find an application on Sourceforge or Freshmeat which you never knew about, and get you interested in things like graphics and music creation.
There are a few decent science programs with which you can explore the wonders of the universe.
There are LOTS of development tools available, if you have any interest in learning to create your own applications or expand existing apps.
I don't think there's a lot that can't also be done with other operating systems, but free tools seem to be generally more plentiful for Linux.