Get a bunch of people who are somewhat involved in an open source project to lead and help new people make some contribution to that open source project. Examples of projects that we can probably get people to help out:
- OpenStack
- MediaWiki
- LibreOffice
An evening of attempting to make esoteric programming languages do some things.
Something similar to https://blogs.dropbox.com/developers/2013/07/introducing-the-python-bee/