Just made some yesterday. I heard to add some sour cream, so I tried that. I always add a green vegetable and extra "real" cheese to the box mix. I don't buy them but I take them home from the food shelf where I volunteer, when they are a little past the date we can hand them out. I also added some ham, and some sauteed onions/garlic.
My stepmother used to make the best baked mac & cheese, she would put in a whole stick of Cracker Barrel cheese, grated. It's so easy to make from scratch.