Nelson's Column in Trafalgar Square is surrounded by wonderful places to visit. The National Gallery; a walk west down Whitehall will take you to Horse Guards Parade, then by No. 10 Downing Street, ending up in Parliament Square. East on foot will take you to Piccadilly, Soho etc. Trafalgar Square is the point from which all distances are measured in the UK so it truly is the heart of the country.
Trafalgar Square is one of my earliest memories of London when we would visit as a kid.
Nelson's Column is great place 2 go :)
Major London landmark. Just be aware that you're not allowed to climb on those lions!