I love lemon vinaigrette, and it's the easiest thing in the world to make -- provided you really want a simple one as you said.
I don't have any exact measurements (sorry!) because I really just do it by taste each time. I use this combination for green bean salads, leafy green ones, tomatoes, new potatoes... any crisp-tender vegetables I've cooked to serve as a salad. I like mine quite lemony, so I do about 3 parts lemon juice to 1 part olive oil, but you should play with the proportions of everything to find what you like best.
your best extra-virgin olive oil
fresh lemon juice
a pinch of sugar
dash of salt
ground black pepper
finely minced herbs, maybe dill, oregano or basil
"What contemptible scoundrel stole the cork from my lunch?"
By W. C. Fields