I love helping people with ideas about London. Please ignore if the last thing you need is more advice HAHA. Here were some of my favs in London and Windsor:
--WIndsor Castle: Take the tour. You get to see a lot, including many rooms you've probably seen before on TV. Get out and see Great Windsor Park by the castle. It's not uncommon to see a member of the Royal Family on horseback.
--Trouping the Colour (if you happen to be there on June 8): iI's the Queen's birthday parade down the Mall, in front of Buckingham Palace. When we went in 2012, we say all the senior royals in the parade. Afterwards, they all appeared on the balcony, while 1000s of Brits showed their patriotic furvour. Was a lot of fun!
--National Portrait Gallery
--Churchill War Rooms: Underground bunker system where Churchill directed the war. Must see, if you love history.
--Tower of London: You can see the Crown Jewels
--Afternoon tea at Clairidges: fun to dress up for before heading to dinner and a musical
--St Paul's Cathedral: Massive and beautiful
-Westminster Abbey: Tour late in the afternoon, then stay behind to attend an Evensong service. About a 30 minute service of music and short readings. Casual dress is ok.