What's an appropriate amount to give to a particularly considerate waitress, who is friendly, remembers the little things, and goes the extra mile. For example, when I was sick, she brought me a free hot lemon drink. When I asked for condensed milk in my coffee, she took note and now brings it to me every time even if she is not in charge of my order.
I don't want it to seem stingy, but I don't want it to be a big deal. Just a low key thank you to show I noticed and care.