When it comes to dressing, ranch is the only choice for me. As far as additional ingredients are concerned, I rather enjoy a little bit of diced boiled egg and some shredded cheese (not too picky about the kind). Diced meat like ham, turkey, or bacon bits are fine, but I wouldn't consider them a necessity.
The only thing that's really a big iffy with me is onion, because red onion is typically what's sliced or diced into salads, but they can be incredibly strong and hard to eat.
Otherwise, stuff like tomatoes, olives, nuts, seeds, peppers, radishes, beets, no thanks.