Circling the entire coast of the Island would be approximately 15 hours of driving. I suggest you follow the Central Coastal Drive. It takes about 4 hours of non-stop driving to complete and gives you time to stop at a few sites and enjoy them without feeling too rushed. I would suggest that you grab a map of the Island upon your arrival at the Visitor Information Centre on the corner of Water St. and Prince St. in Charlottetown. It's walking distance from where your cruise ship will dock. You could also ask a vacation planner there to alter your routes in order to cut on driving time to get the most out of your day on Prince Edward Island. I also suggest booking a car rental in advance. Doing a self-drive tour to really experience the area at your own pace is easily done.
The South Shore and the red beaches are my personal favourite! Canoe Cove
itself can keep you busy for a good portion of your stay, exploring its combination of endless sandbars at low tide and rugged red shore on higher tides. There's a nice population of seals along this portion of Prince Edward Island, so bring along some binoculars!
Argyle Shore Provincial Park is just up the road (which is also a great place to explore at low tide) and in the other direction, you're only minutes away from Skmaqn–Port-la-Joye–Fort Amherst National Historic Site.
If you continue travelling along you will reach the seaside fishing village of Victoria by the Sea. It's a charming little village with lots of artisans in town, a chocolate factory and its own playhouse. It's also a great place for lunch! After Victoria, you can continue along the south shore to the Confederation Bridge.
From there, you can continue the Central Coastal Drive towards the Cavendish region where you'll find our beautiful National Park as well as the famous Anne of Green Gables Heritage Place. This loop will continue along the north side of the Island and back to Charlottetown.
Enjoy your day on the Island! I'm sure you will find lots to explore!