My son is a mac'n'cheese fiend as well, but fortunately his definition of mac'n'cheese (Rocky Cheese) is pretty broad. Here's my super-quick standby: cook some maceroni, and while that's cooking, nuke some frozen spinach (or broccoli, or peas, or whatever). Put a little butter and plain yogurt in a big bowl, add the hot veggies, the hot drained pasta. Throw on some grated cheese (I like spinach with parmesan, but any cheese with whatever veggie is good). Sometimes I'll also heat a can of beans and throw it in as well. Mix it all together.
I'm also a big fan of cooking a batch of brown rice, and using it in different ways. I love my crock pot, and chicken soup from that can keep us going for several meals: one day add some pasta and frozen peas, the next day add brown rice and broccoli, etc. I just take the meat off the bone right away, and then use the broth and chicken as a base for everything.
I also like to freeze some of a soup, or spaghetti sauce (I have to make my own, I dislike the canned stuff), and then pull it out, so I don't get sick of any one thing. Sometimes I'll get ambitious and do pizza--parbake the crusts, and freeze them. Then I just pull them out, smear on a little sauce, put on some cheese, bake it.
We do a lot of veggies (cukes, zucchini, and carrots) and "dip" (mainly plain yogurt). Crackers, mustard, and cheese (ok, probably most kids wouldn't go for that, but my boy's a weirdo
). Grilled cheese sandwiches. We're big on frozen vegetables (with my schedule, it's frozen vegetables or no vegetables, sometimes).