Mixed salad can be varied infinite ways: you can add, remove, or change anything you like. Follow your imagination, or what you have in the fridge. The basic principle is this: mix fresh ingredients cut small with herbs and French dressing.
From there on, anything goes. Some suggestions: small cubes of cheese (feta, gruyère, cheddar), bacon or ham, cooked potatoes, etc.
Same thing for French dressing, you can use the classic one, or change the oil (sesame, walnut, ...), vinegar (balsamic, cider, maple, lime juice...), or add other tasty things: teaspoon of soya sauce, dash of Tabasco,... Even all of these!