While having plenty of games and activity generally (lots of nice ideas there), try to put aside a space for quieter play for littler ones especially. If you have a few small baskets or bowls of crayons, free printable colouring pages (favourite characters and lots of Christmassy ones are available on the net) and maybe even some seasonal activity sheets (like wordsearches or sudokus) for older ones, that would be good.
Maybe do a treasure hunt sheet too? Might need to have a younger and older version of that. Either like above suggestion, or find lots of different things like
Find the star stamp and stamp this line.
Get Mrs Moo from Milk Street/John the Butcher/the local headmistress... to sign here (someone you know will be there)
How many windows are in the hall?
(OK not particulary inspired this morning, but I'm sure you'll get plenty of ideas here, elsewhere on net or among locals)....
Bring along an ipod or cd player and speakers, with Christmas music for a disco. If anyone has a glitterball or any disco kind of lights, get a loan of those too (lots of people now have small ones from Argos etc, don't need to be real "pro" ones).
If it's not too chaotic, a craft activity can be good to settle kids down. The local GAA club hold their kiddie party in a school, with stations in 3 different rooms and people move between them all in groups - making rocky road type buns in groups of about 2/3/4 (each crushes 2 digestive biscuits in sandwich bag, can choose how much raisins/marshmallows to add to mix, all get to mix into chocolate melted by grown up helpers in large bowl, and then spoon into 2 paper cases each), making paper plate wall decorations (coloured in, small pictures decorated and pritt sticked on, stickers and glitter glued around edges and all 3 plates stapled by grown up helpers to a ribbon), getting a snack (apple, digestive biscuits and diluted squash), and all come together in the hall to meet Santa (who tells lots of jokes and funny stories, getting them singing along etc, but there aren't any gifts handed out there).
Will the Hall already be decorated? If not, get plenty of tinsel, crepe paper chains, can of snow/gold spray etc and make it as magical as you can.
If snowy, make sure plenty of space to take off and store outer layers, and hot food or drinks close after as people come in.
I agree with things like carrot sticks and chopped fruit as options - kids often choose at least some of those when they're offered, even at parties. And yes to asking people to bring things - and try to ask them to bring something specific (so you don't end up with 40 plates of ham sambos and nothing sweet, or vise versa). If you can, either have washer-uppers sorted beforehand (not knowing if this is somewhere that has all the crockery needed), or else buy a catering pack of paper plates (even if crockery there, if you fear you might have to do all the cleaning after too and especially as excitable kids can mean breakages unintentionally).