I went to my first retreat a few months ago. If you can't take a chair, take a pillow for the provided chair because you may be sitting in a folding chair at a lunchroom type table.
Take some earbuds for your phone or other device that can hold music. If I go to another retreat I will remember to take MORE breaks and use my earbuds more often (there were no classes at this retreat). I live alone and mainly an introvert so I'm not used to being around people 24/7. It was overwhelming for me and sometimes I just wanted to focus. Others also had earbuds to tune out the group when needed
If you wear glasses, take a spare pair.
I always travel with earplugs so if there is noise in the hotel room, I can block it out. I offered them to my roommate since I know I snore
Extension cord, power plug, travel iron and small pressing surface.
Water bottle or coffee mug that can be closed.
Along with prescription meds take OTC meds you may use - ibuprofen, tums, etc.
Projects - take a large piecing project, some small projects - like you mentioned things like bowl cozies. Christmas ornaments are a nice small project. I did a couple of dog coats while I was on retreat.
I put my projects in large ziploc bags so I could take them out and put them away in some type of orderly fashion.