Grapes/melon/strawberries if you have to take food, or the small tubs of Haagen Dazs ice cream are about 12.5/13 syns depending on flavour! Maybe just take a drink and enjoy the movie? What you watching? Just being nosey lol
A bag of metcalfes sweet and salty popcorn is 6 syns and you get quite a bit for that. You can get them from Cafe Nero and Gregg's but they had a sharing bag in Poundland the other day that had 3 servings in so worked out much cheaper.