Go to Hobby Lobby and check out what they have there. They have a large variety of poster board, stand-up display boards, etc. You could use some peel and stick letters to make a headline...explaining what the event is, who it's for, etc. You could also have a variety of permanent marker colors (probably fine or medium point) for everyone to sign and leave a message. Their staff is very helpful and I'm sure if you described what you are making they could help you pick out your needed items.
Good luck...and HAPPY NEW YEAR!!!