The old city is small, walled, and largely pedestrianised — you will not need any transport inside the walls. From Canterbury West station to the cathedral is a ten-minute walk; from Canterbury East it is five.
If you are staying near one of the universities or out toward Whitstable, the local Stagecoach buses are frequent and cheap. Day passes are good value if you intend to combine the city with a coast or countryside trip. Taxis are easy to find by the bus station and Canterbury West.
Cyclists will find the city itself friendly enough, though the medieval lanes are not bike-priority. The Crab and Winkle Way is the most pleasant local ride — a marked rail-trail route along the line of the world's first commercial passenger railway, running seven traffic-free miles from Canterbury to the beach at Whitstable.