Salmon - tuna leaves a terrible taste in my mouth. Other than that, chopped celery, relish, mustard, ketchup and Miracle Whip. If I have some bacon cooked and leftover, I will crumble that into it. Never raw onion as that also leaves an awful taste in my mouth. Never mayo because I don't like it.
I use the same set of ingredients with leftover chicken and pork My DH has to make his own tuna salad!