Windsor Castle is only 20 miles outside of London, not far from Heathrow Airport. It can be reached by train, bus, or car, or you could join a tour. As the oldest and largest occupied castle in the world and the primary residence of the current queen, it is definitely worth a visit.
The castle is very visitor friendly; there is a good historical overview in the first building you enter. You may purchase an official souvenir guidebook or rent an audio guide at the start of your visit for a self-guided tour. In addition, there are 30-minute guided tours available for free.
Things to see in the castle include the lavish staterooms, the gallery featuring the royal art collection, and Queen Mary’s Dollhouse (which I didn’t see). Downhill from the castle is St. George’s Chapel and the Albert Memorial Chapel. We got to see the changing of the guard as well. Outside the castle walls, the city of Windsor abounds in shops and restaurants, etc., which visitor might enjoy.