Not been in many years, but spent a large part of my childhood watching people row and sail back and forth. It freezes over in winter and in February 1893, Jack Selby drove a coach and four horses across the reservoir.
The AA is Britain's oldest school of architecture, but anyone can take advangte of the great work going on there. You can catch really interesting public lectures and free exhibitions throughout the year. There are also occasional installations and event launches, a bijoux cafe and brilliant bookshop. A great place to hang out with the builders of the future.
A great place to start the day if you are in South London is Federation Coffee in Brixton Village. The food is great and so is the coffee. This is a very chilled place where you can get both and indoor and outdoor experience (even though the whole place is under the Village roof).
The Thames is not traditionally the prettiest river but it has its moments. Here it is turning pink in a sunset this summer. Its the reason the city is here and I like how in moments of real importance it rises to the occasion. For the golden Jubilee the queen floated down it with hundreds of boats, it was an amazing sight, like vikings setting out for battle. Good river spots - North Bank Savoy Gardens, Royal Festival Hall.