We do this with a friend every year. Past ideas have included:
a family board game with snacks to eat whilst playing
Mexican night, with tortilla chips, chilli sauce, jalepenos, salsa etc
Baking camembert in a pot, with posh crackers, pickles, wine etc
Sundae glasses, syrups, sprinkles, flakes
nice beakers, twirly straws, nice cordials
a fondue set, chocolate, marshmallows etc
movie night - film, coke, popcorn, pick and mix