Richmond Sacramento Salt Lake City San Antonio San Diego San Fran. Participants are required to bring a gift valued in a specified range, which is wrapped, but not labeled. Include the rules on the invitation because this will entice people to come and participate. This also gets their creative juices flowing. The only time I've experienced this theme is during the Christmas season, and most people have xmas decorations out already.
Snack food is very appropriate.
You can even ask your guests to bring their favorite holiday snacks. I usually put out a fondue pot with mushrooms, diced chicken, beef, shrimp, cheese. I also put out chips and dip and a vegetable tray of broccoli, carrots, pickles, monteray jack cheese and cauliflower with veggie dip. You can include cookies, cakes, swedish meatballs, etc. If at xmas time then holiday music, if summer maybe beach boys, it's your choice.