Fix the indentation in the JDK source code.
Many parts of the Java source code that ships with the JDK are very poorly formatted with respect to indentation. Take a look at the Basic UI classes for starters (BasicScrollPaneUI, BasicInternalFrameUI, etc.).
The lack of indentation makes the code very hard to follow and this always gives me a bad headache, especially when developing things in Swing which frequently requires knowing the JDK source code.
It's such an easy thing to run the source code through a formatter to improve its readability, so I think this is little effort, big gain.
Of less importance, opening many of the JDK source files in a decent editor (like IDEA) always reveals a load of redundant imports in the JDK source code that could easily be tidied up.