My friend Sammy was here from the States for my birthday. We took one day to day trip to Canterbury. It's an easy day trip from London. About £20 and 90 minutes on the train out of Victoria station, which offers plenty of departures and returns. You can easily cover the key sites of Canterbury and still have time for tea in a day.
Our train conductor had suggested we make a point to get all the way to the West Gate - it's that the end of St. George's Street, which turns into St. Peter's Street about halfway down - for the views of the city. It was good advice.