Bottled water, unless that is provided. Nice to have next to your sewing area instead of going to a fountain. Otherwise, you seem to have it pretty well covered. I'm sure others will have some ideas for you too. Have fun.

What you are bringing sounds good.
When my guild has its 2 day retreats, I also like to have:
Tools: machine oil and cleaning brush, an extra blade, extra camera batteries. Be sure you have bobbins and thread for your projects.
Are you planning to do binding? Will you need hand sewing needles?
I like a cushion for my tush because our chairs are very hard.
Food? Drink? We generally brings snacks as well as have a pot luck towards the end. Be sure you have whatever you like to drink. Where we meet, we need to bring plates, cups, utensils as well.

bring a UFO to finish

have fun and be sure to make you up a check off sheet so you don't forget anything

If you need to take medications, make sure you have those.

Don't forget to bring an extra sewing machine needle in case you break one.

ME!!!!!

Me, too

Double check to make sure that you have your cord and pedal for your machine. I've known several that went to a retreat or class and had forgotten them.

Ditto on that one boy did i feel stupid

A pan of brownies, a bag of chocolates, a bag of tangerines, basically anything easy and share-able. Bottled water, your sack lunches unless that stuff is provided.
Take two or three UFO's to work on and the things needed to finish them. That way if you get tired of working on one or get it finished you'll have something else to do.

Small baggie to tape to the table for garbage (unless you have a little tidy tote with a pin cushion
Show and Tell, Name Tag, handwork, book, Masking Tape
Camera and batteries, and recharger and chair.
Commuter mug
Paper scissors, snips & fabric scissors and seam ripper.
Post its
Notepad & pen/pencil (pigma pen for labels)
blue disappearing pen, yellow or white chalk marker, extra fabric in case you want to make something else?
Fabric for projects you are working on. Batting
Sewing machine (including your foot pedal & power cord, manual, 1/4 inch foot, walking foot, free motion foot, screwdriver)
loaded bobbins
new blade for rotary cutter
Threads for piecing and quilting.
Ott-light, magnet,
Stabilizer (newsprint or tissue), heat n’bond lite, freezer paper
Safety pins, straight pins, basting spray, thread catcher
Wrist brace
Design wall

I would take a package of machine needles in case you break more than one. Hope that doesn't happen, but you never know. Bottled water and snacks are good too. Also take several spools of thread in your color choices. You may also want to take some hand sewing needles and a thimble if you plan to finish your binding by hand. That way you just might come home with some truly "finished" objects.