Make your own mac & cheese! It's yummy, and marginally better for you.
(My family's mac & cheese recipe is always my favorite...it involves sharp cheddar and fat-free cottage cheese and yogurt. Om nom nom.)
Ahem. In a pinch though, with the boxed stuff, I'd cook it up with some diced fresh tomatoes and some pepper. A little bit of garlic (fresh and minched, or garlic powder if you don't have any fresh) for flavor would be good, too. Throw in some chopped up, cooked ham or ground beef for some extra protein; mac & cheese & ham is a good combination.
I definitely wouldn't add any additional salt, since boxed mac & cheese tends to have a lot of it on its own. Experiment with some other spices for flavor instead.