London has so many things to offer. They will be spoilt for choice specially in summer when the weather is good.
Option 1 What I could recommend is maybe offer them Southwark experience. We will do a little walking tour of the Southbank ; William Shakespeare connections, will see the sight of the old Globe Theatre, the Theatre reconstructed by Sam Wanamaker in 1994 . If they want we could do the walk in medieval London and see the vile Clink prison ( lots of torture and execution). We could go and see Tate Modern Gallery with newly opened wing called Switch House, highlights of the gallery ( unless they would like to go in details). Visit to famous food market Borough where they can have a glass of wine and fresh oysters.
Option 2 would be Soho Snd Covent Garden walk. It can easily be combined with the visit of British Museum ( or if they would like something unusual then the visit to Soanes museum ( 1837 Appartments preserved as it was left by the architect) its open Tuesday to Saturday only, free of charge.
Option 3 legal walk of London we will explore London Legal Quarter between Temple and Lincolns inn fields. If they would like, we could enter Royal Courts of Justice beautiful Victorian building, free of charge as well.
Option 4 Walk in Maritime Greenwich taking the boat from Big Ben we will sail through to Greenwich, will see the Royal Observatory Prime Meridian and stunning Royal Naval College and the Chapel. Can also visit Nelsons Tavern for a typical Pub experience. Visit to Greenwich Market.
Option 5 Classic London.we will start our walk from Trafalgar Square 0 Mile the country, will go down the Mall to see royal residences including Buckingham Palace main residence of Queen Elizabeth II. After a short coffee break in the bark we will continue on to Partliament Square to see Big Ben and then Downing Street to finish off at Piccadilly where there are lots of options for lunch.