Kraft Mac N Cheese isn't bad, unlike the store/generic brands.

Since college I've been making Tuna Mac from a recipe I saw on the box years ago. It's pretty good, and slightly modified by myself:

To prepared Mac N Cheese, add (while simmering on low):
1 can condensed Campbells Cream of Mushroom (only thing Campbells is good for)
1 can or pouch of tuna (with its water; oil is gross!)
1/2-1 cup frozen peas (small can if you must); I boil the peas w/ the macaroni
Sometimes I'll add a sliced-into but whole clove of garlic into the boiling water, then crush it into the dish when mixing it together
Some additional milk to proper consistency (about 1/4 cup)
Sometimes I'll melt a slice or two of American cheese in there to make it cheesier

Season to taste
Heat and Eat!

I tried it with a can of salmon once, and it was interesting. A bit too much, though.