If you choose carefully there are lots of low calorie party snacks, (calories are per item)

olive 5 cals, twiglet 4 cals, king prawn 5 cals,  1 tbsp guacamole 25 cals, 1 tbsp tomato salsa 10 cals, 1 tbsp tzatziki 20 cals, slice of ham with no fat 20 cals, slice of wafer thin ham 7 cals, cocktail sausage 30 cals, pickled onion 3 cals, tortilla chip 12 cals, 1 tbsp of salted peanuts 190 cals, 1 tbsp hummus 45 cals, 1 tbsp taramasalata 65 cals, slice of garlic bread 90 cals, mini pork pie 175 cals