Walk through the Park to the Observatory, take your photos standing on 0 degrees longitude, enjoy the view from the observatory hill, maybe even go in the aforesaid observatory. aside from the huge national maritime museum, there is the beautiful Queen's House complete with murals painted by Artemesia gentillesci (an't spll her name), an artist 'forgotten' by art history on account of being a female lady. walk back past the (charred remains of) the Cutty Sark and go down the tunnel under the Thams, walk under the thames to island gardens and get the Jubilee Line just for the heck of it back towards the Tower and go up the Monument to the Fire of London
http://www.themonument.info/