London is a big city. There are many places that people can go to, so you need to plan your trip well if you want to visit those places.
If you plan to stay in London in a few days and visit many places, it is recommended to get a London Pass as this provides free entry to various attractions.
Westminster Abbey is a must visit if you are interested in the history. It is a place for coronation and burial place for English and British monarch. You can also see the tombs of famous British including Sir Isaac Newton and Charles Darwin.
Based on my experience, metro is the most reliable transportation as it is quick, no traffic jam, and the metro lines are quite clear.
Hop on hop off bus tour
This is a great tour to explore London. Try to avoid high peak hours when doing the bus tour, otherwise you may be trapped in a traffic jam for hours.