Ask HN: Coding Projects for high school grad with Java knowledge?

A friend of mine asked me for some advice regarding her son, who recently graduated high school and will be going to a good university seeking to major in Computer Science. Her son has completed an AP Comp Sci course that focuses on Java programming. Unfortunately, the summer job that he had lined up fell through at the last minute. I figured that maybe he could use the time to get some skills that will help him look for an internship after the first year. I have provided him resources to learn git and unix. I am looking for ideas/resources for small coding projects that he can work on this summer, maybe even any open-source projects that are feasible for someone with some Java knowledge.

Any ideas will be welcome!