Prepare macaroni and cheese according to package directions. Transfer to a 2.5- to 3-quart baking dish.
Preheat oven to 350°F.
Stir undiluted cream of celery soup and milk into the macaroni and cheese until smooth.
Gently fold in drained tuna and optional vegetables, keeping some tuna pieces intact.
Scatter crushed potato chips evenly over the top without pressing them down.
Bake for 30 minutes, until edges are bubbling and chip topping is golden.
Let sit a few minutes before serving.
Notes
Drain tuna well so the casserole does not get soupy. Fold tuna gently so it stays in pieces instead of turning into paste. Do not press the potato chips down because they crisp best sitting on top. Cream of mushroom can replace cream of celery if preferred.