|Where to begin........
Night shot of Tower Bridge I would put at the top of my list. With the sun being low in the sky this time of year, weather permitting you could get some great architechture shots, (long shaddows, high contrast, that sort of thing).
At the right time of day the Houses of Parliment from the southj side of the river, (Westminster Palace) would be one of my goals.
Also there's a no so well known landmark simply called The Monument. Its on the north bank of the river just before you get to the Tower of London. It stands near the sport where the great fire of London started, Pudding Lane. You can go up to the top and get some great city view shots.
My advice is take plenty of memory cards or a laptop to download onto, there's so much to shoot in London.