Elementary school is so hard.... they all know who is invited and who isn't, but I always said all parties are NOT co-ed to my children. So I only had half the class to worry about each time and I always invited all of them.
I liked giving a thank you card with the gift bag, because we are horrible about doing them afterwards, so I just gave a small candy, a pencil or stickers or maybe fun chapstick... or sunglasses.... depending on the birthday month and a note that said "thank you for coming to my party!". There are so many themes for you to choose from.... ask your daughter.
Chuck E Cheese has party bags too, don't they?
Enjoy and try to get keep the level down on how much you do for parties, because your childs expectation grows each year.