Any river journey would be good, but obviously a RIB trip should be top of the list! Try to get on the longer trip that goes to the Thames Barrier, it's well worth seeing.
How about a more sedate trip upriver from Westminster to Kew or Hampton court?
The Museum of London Docklands and the Maritime Museum at Greenwich are both worth a visit (take the Docklands Light Railway).
HMS Belfast, the Tower of London and the Tower Bridge Experience are good too, and all close to each other in Central London.
If you want some good views the London Eye is good. The Shard is better though, massively higher with a panorama of the whole of London although further from the river. The Emirates Airline cable car from Greenwich Peninsular to the Royal Docks is great for an alternative river view as well.
That should get you started. How long have you got?