If you find yourself staying near the Rato go for a walk around the little Amoreiras garden under the aqueduct. Then tucked away up an alley in a corner there's a little exquisite bar not many people know about, called the cornuncopia. Very cosy and private. You have to ring the bell.
As a woman, I have to say that if anyone propositioned me anywhere in Alfama I'd find that stunningly romantic. Especially if it was from somewhere with a view over the river.