One of the trips you can chose to take using a London Pass is to go to Windsor Castle, the family home of Queen Elizabeth II and the burial place of King Henry VIII among others. At first I was a little hesitant to go because it actually isn't in London proper and you have to take a national rail train out. But, it turns out to be very easy and a great trip for a Saturday.
Once in, I really recommend seeing the changing of the guard at 11 am on most days. They shout a lot which is funny and play modern songs: they played the frozen movie song “Let it Go” when I was there!
Overall, I had a great time. The trip took less than 90 minutes and then I spend a nice 2 hours or so in the castle. I would recommend it if you have time and a London pass, but normally it would be pretty expensive since the tickets are £20 and then the train would be another £20 probably.