There is a lot to see and do in London, enough to fill more than a couple of days. I would consider it a pity if someone had been to London and not seen some of world famous musems -  the British Museum (even briefly), the Natural History musem. Also I would recommend the Tower of London (plus the houses of parliament), Westminster Abbey, Tower Bridge. London is also world famous for its theater and night life,  I would suggest making an effort to see one show at least. You can check out