From what I understand, the food needs to be ready to eat, not something that needs to be prepared, so I think that the spaghetti idea is also out. I could be wrong, so someone could correct me because I see layouts in magazines with jars of soup ingredients that you just need to put in a pot and add water, or cookie recipe that you just need to add a few small ingredients, or such things, and I’m never sure if that is allowed, but I thought it wasn’t.
I had a friend who made some kind of tray that had places for a few small round cups (think ketchup or salad dressing from a take-out restaurant) and she put a few different kind of shmear salads in them and had crackers in the middle. The whole thing was shaped like an artist’s palate and her family was dressed up as artists. That is what is meant by a theme, and I think this is a pretty inexpensive way to go.