As a developer I regularly try to attend hack days. They offer a chance to meet other developers, people and companies in your community; work on something interesting and potentially win some great prizes.
Here is a basic kit list for successful hacking:
Most importantly you need your laptop. Make sure it is loaded with everything you might need prior to attending the event. Not only does it save time, but it also saves you the agony of trying to download and install software on an internet connection which could be worse than rubbish.
3G/ 4G phone
It seems to be that even the best organised hack days cant seem to keep the internet up and running. If the internet is important for your project make sure you can tether your phone and computer. It will keep you running whilst those around you are struggling to get things working.
At many hack days the organisers will offer a cabled network option. Be prepared take a length of ethernet cable. If you think the wireless is going to go down or be unreliable get your ethernet cable connected before others get there.
Before your leave home ensure you have all the software installed and configured on your laptop. Also ensure your have downloaded any documentation you may need.
At a hackathon there is a good chance you will end up like this at some point.
A small pillow is a nice to have and highly practical at most hackathons. There is the chance you will be sitting down for hours working on a project a pillow allows you to keep comfortable as well as offer a place to rest your head if you need to 10 minute nap.
If you are doing a hardware hack then hopefully it speaks for its self you need to take some hardware with you. For hardware hacks its recommended to take slightly more hardware than you need to ensure you have what it needs to complete your hack.
List of Ideas
At most hackathons I try to go along with at least 5 ideas for projects. Having a hack list helps you started coding sooner and allows you to form a team easier. It is also nice to share ideas with other developers who might be struggling to come up with ideas.