I've been to a retreat twice and loved it. We had a supply list, because we also would get taught different things.
Irons and boards were provided.
I would recommend some type of pain killer, like tylenol or so. Sometimes your eyes get tired because of the strain.
Bring a notebook and pen, to take notes, you may also end up making new friends and want to stay in contact with them.
Also dress in layers, sometimes it could be cold, so you can take your clothes off or put a light jacket on. May be you also want to put some warm socks on. (May be you don't need it, because of the warmer weather.)
Either way, dress comfortable, you will sit a lot. Bring your favorite coffee cup for your coffee or tea. May be also a nice picture of your loved ones.
Most of the time you will have both, time to work on a project together and also time to spend by yourself.
The best way, may be you could contact the people who put the retreat on and ask them what is necessary or a good idea to bring.
Most of all, enjoy and have a good time. For a little while forget the everyday stress and just chill out.