I have lived in London for a while now, and to be honest, it was an embarrassing number of years until I discovered the gem of St Katharine Docks. This place is so central, but so easy to miss. The first time I stumbled upon it, it was such a nice surprise. Not only is the place beautiful, but it is a nice spot for a drink or some food.
I'm going to say no more, except if you haven't yet been - make sure you find it!
Location: Closest tube is Towerhill