Fabric
Pattern
Thread
Extra Needles
Marking Tools
Cutting Tools
Back-up project in case you finish or get bored with the one you are working on.
Seat cushion (or even your own chair if you have doubts about what will be available)
Clothes (outer and under)
Medicines (if you need them)
Cell phone charger (even if you leave the phone in your bedroom, it is nice to keep it charged)
Snacks if not provided by retreat (or if you have dietary needs)
Cup with lid
Cash in case you want to go out to eat with friends.
Hopefully others will chime in as well (I feel like I've been killing threads today).
Cheers, K