Going for a walk along South Bank is a perfect way to see London. We would recommend about an hour walk (although you can make it shorter or longer!), where you can see a lot of the major sights, and stop at some pubs along the way. We've posted a link here for an example route.
We have tagged some recommendations for things to do in London here. Purple tags show site seeing activities, orange tags show pubs/bars, and blue tags show restaurants.