Game jams (similar to hackathons) are great for applying what you already know but with a new group of people. They're much harder if you _dont_ know what your doing (contrary to what you suggest).

I've found that the group really makes the experience, and find them less fun for the tech and more fun for learning about the people through the work and a team project without the constraints of a corporation.

And generally, it's not just people who aren't tied up in this that will participate, but people who will make time to do something exciting and form an interesting connection.