My salad dressing is pretty basic, but it's at least 90% of the point of eating salad for me - and 100% the point of eating lettuce, the only point of lettuce is to give your salad dressing crunch & make it easier to get into your mouth ;) I basically just mix about equal quantities of olive oil & vinegar with a pinch of salt & pepper. I started off using a variety of vinegars, a different one each day - normal malt, balsamic, white wine, even lemon juice ... whatever I had in the house. But I've settled down into just using balsamic vinegar coz it's my favourite. I probably wouldn't serve anyone else salad in that dressing though, it does turn the veg a rather disconcerting brown colour ;)
Veg wise I stick to lettuce, tomato, cucumber & spring onions. I keep thinking I'll branch out (be wild! ;) ) & have pepper or brocolli or something, but then when I get to making the salad that never seems as tasty as the standard set of ingredients. I put some leftover roast chicken in once recently, but that turned out to have a disconcerting texture amongst the veg - a few "omg what's that?" moments.
Of course, the other reason I end up with always the same veg, always the same dressing is that I'm very much a creature of habit when it comes to food. I'm happy eating the same thing for lunch every day, and while I prefer more variety in my evening meals I always quite like the same thing on the same day of the week. It drives J nuts, I think ;)