I love retreats! Just went to one a few weeks ago, and am ready to go again. You are going to have so much fun.
I always take extra thread and extra rotary blades. I also take a fan. It's saved me many times when I'm flashing! I have one of the little ones from Walgreens that costs $10. On the other hand, if you're one of the few who are always cold, be sure to take a sweater. If you have two machines that travel well, take the second one, too, just in case your machine has a problem. I always take my Bernina and my Featherweight. Even if you are pre-cutting your fabric for your projects, take the uncut fabric, just in case you make a mistake. And your fabric may be just the right color that someone else can use.
If you're doing a group project, take extra. The last retreat I went to, the pattern called for a 1" piece of velcro. I just took the whole package, and shared with at least 3 other people.
On the personal front, don't forget your meds, your phone charger, an extra pillow. If you sew barefooted (as many of us do), be sure to take some warm socks or footies. The floors are usually cold.