Many people at the EAGx Berkeley Hackathon chose to contribute to open source projects, which is a great way to learn and practice software engineering skills - not just programming, but project workflows like using version control, static analysis, testing, and responding to code reviews (it certainly worked for me).  This page goes into more detail on how and why people contribute to OSS projects.  However, to calibrate your expectations: contributing to open source can be challenging, and getting set up is often time-consuming.  

I therefore recommend running through this interactive intro to GitHub and then this quickstart guide (up to "be social") well before coming.  If you don't have previous experience in open source or working in industry (few academic labs teach these skills!) you'll probably spend most of your time learning the process of contributing, rather than writing code - but it's worth it!

Here's a link to some specific open source issues.

Almost all projects started at hackathons die young, so if you want impact I think you should contribute to existing open-source projects. This is also a great way to develop software engineering skills, which are a key bottleneck for AI Safety research among other fields, and I think a day modelled on PyCon sprints would be a great hackathon experience. 

I wrote up a list of specific issues here.