I use green onions, frozen peas, cheddar cheese, black olives, celery, hard boiled eggs, mayo, a touch of dry mustard, sweet pickles, along with some of the juice, al dente elbow macaroni... . If this is for a party or gathering I sometimes added baby shrimp.
One thing I believe made this a hit is that I used enough hard boiled eggs, in fact I always used three egg yolks without the whites in the mix. I always made up my wet ingredients and mix well before I ever added my chilled al dente pasta. Never serve before it gets a night in the fridge, it firms it up good and flavors build upon one another.