My DD needed a bathing suit at CVW in January because track ended up holding practice in the pool. Pillow and sleeping bag are also helpful as well as snacks for your plebe and roommates. It has not been warm here very much, especially at night, and you will probably walk a lot.
Most importantly this is your chance to ask lots of questions so take advantage of it and prepare a few. Try to talk to the mids at your briefings and the professors after your classes. Don't just observe, participate. If you are a junior, find out what classes will really help you next year, not just what will "look good". If you are taking APs, find out if you can validate some classes plebe year or if you want to take them again. You will be able to tell by the classes you attend.
You have been offered a great opportunity. Good luck and have fun!