The Cathedral is a very large complex and well worth the admision fee as you could comfortably spend a few hours exploring it. For me it was also a deeply spiritual experience, I am not a religiouse person but the poigniancy of the areas associated with Thomas A Becket really affected me.
The other sites inscribed on the list are not to far away either they are about a 10 minute walk from the Cathedral, The Abbey has an entrance fee but was closed when i got there at 5:30 but you can still look in, St Martins church is a few minutes further on and is quaint, but is impressive as it is the oldest Church in Britain.
The town of Canterbury itself is really lovely and has much more to offere than just the Cathedral, it also has a much more European feel as it is easily accesable from Dover and Ashford so in easy range of both ferries and Eurostar Trains.