I usually make mine with garlic, milk, and bacon. Or ranch or Caesar salad dressing if I don't have anything else to add in.
If I'm having them with a traditional holiday meal then my stuffing, cranberry sauce and vegetables usually all get mixed into the potatoes- on my own plate, guests are free to do as they please with their mashed potatoes.
This may sound kind of gross, but when I make instant mashed potatoes I add in mustard. Not a whole lot, but if you add in a little into the Costco brand that my parents sometimes buy then they taste really good.