What to work on
You probably already know exactly what you're interested in working on! Whatever you decide to work on, please post a message on the scala-ide-dev list to see if anyone else is working on anything related and collaborate if possible.
If you are looking around for things to work on, have a look at the Enhancements milestones. We listed a number of well-delimited improvements that are nice-to-have and nobody is currently working on.
See Open Tickets for further inspiration.
Last, our Regression Test Suite could be expanded. Creating tests is one of the best way to start contributing now and bring immediate value to the project!