Deli ham. I take one or two slices of it, rip it into bite size pieces and mix it into your mac & cheese. Suddenly it becomes much more exciting.
Hot dogs. I've been doing this one all my life. Chop up a hot dog and mix it in. Delicious.
Onions. I tried this one and wasn't a huge fan. But I also used way too much onion. Right now I'm just trying to get your juices flowing. Let me know about your ideas!
Also, if you happen to run out of milk, don't fret. It still tastes good without it. Just add a little bit of water to help the cheese powder mix in better.