We run agency hack days every month or so and have had interest from people wanting to join us for them from outside the company so we thought, hey, why not - let's do it.
Our hack days follow this format:
4pm on hackday-eve (yes, that's a thing) - get together to discuss ideas, collaborations and what people want to do.
10am on hack day - get cracking. Code, design, learn, play. Whatever goes (but please no nudity, that would be very weird).
5pm on hack day - 60 second summary of what you've done and why you found it interesting.
Food and drink is provided throughout the day, we do tend to have a web design/development angle to what we get up to on the whole but if you think you have a suitable idea of what you want to work on, bring it with anyway.
Internet's good (500mbit) and chairs are comfy, desk space provided although we would advise bringing a computer if you need one as we're a bit short on spares.