Allegedly the Queen's favorite retreat, Windsor Castle is a truly magical place. It looks just like the sort of castles we imagine when reading fairy tales of princesses and dragons, and is set across over 13 acres of land in total, making it the biggest inhabited castle on Earth.
Trains set off for Windsor Castle from Paddington Station, and this is one of the most convenient day trips from London as it only takes around half an hour on the train to get there. You'll be getting off at the Windsor and Eton Central station and need only walk for 5-10 minutes to get to the castle.