If you want to have a view of the city, the London Eye is quite good. Book tickets online as the queues can be insane, they are a bit expensive but worth it if you are only going there once.It is also 5 minutes from South Bank which is worth seeing and having a little roll on. There are also boat rides up to Greenwich (I think). They point out a lot of historical stuff and only cost a tenner. There is a bar on board or you can buy drinks and food from a supermarket and sail up and down the river. This is all pretty standard touristy stuff but you can include South Bank and spend a pleasant and fairly cheap day seeing London and not getting too stressed. All within 10 minutes of Embankment tube station.