Charlottetown is a wonderful walking city in pleasant weather. It's relatively small, so you can easily walk the residential neighbourhoods, shopping district and historical landmarks with nothing more under your feet than comfortable shoes.
PEI as a whole is easily navigable by car or bicycle. Personally I would love to return for a cycling trip -- we saw countless cyclists throughout Atlantic Canada, some into speed and some into casually exploring the inviting terrain of the region.
Photo credit: Jason Kurylo