Talk to the parents, they will be a wealth of information for you. I know that with majority of parties we attend, I tend to take our own little cupcake for the cake eating section. You could alternatively bake your own little GF cupcakes (there are plenty of packet mixes available) you do need to be careful to avoid cross contamination though.

We are having DS birthday soon with GF, we will have fruit platters available. There are yummy chips called "vege chips" in woolworths that DS loves, not sure how healthy they are though lol. Popcorn also. I have a small selection of lollies too.