As far as fall party themes... you could take empty baby food jars and have the kids make thumb print turkey's or ghosts on the outside and fill the jars with some sort of candy-- then take a holiday sticker and placed it on top to cover the baby food jar cap.
I guess I would check with the teacher to see what she/he would like you to do as far as activities and check with the PTO or whomever is in charge of appointing room moms to make sure you know exactly what is expected of you and go from there.
At the elementary my children attended the room parent was responsible for coordinating games and treats for parties, helping with Teacher Appreciation week activities, and finding volunteers to work the spring carnival. I'm sure that duties vary from school to school, so if I were you I would find out who the coordinator is for the Room Parent program and check with them.
As for Fall Party ideas, are you going Halloween themed or just fall themed?
I have just been asked to be the room mom for my daughters 1st grade class. I am not sure, but I think this basically just puts me in charge of parties. My first question is does anyone have any ideas for a fall party? Secondly, is the room mom responsible for more than parties? Any room mom's out there have any answers? Thanks!