I have been there but not for an overnight.

Maybe book a package that includes a tour bus and hotel. Definitely do the Anne of Green Gables visit...author Lucy Maude Montgomery and her husband are buried in a cemetery nearby. There are two sites - the one place that is reconstructed as the home and surroundings in the book, and is beautiful! The second is to visit the house where Lucy visited as a child - distant relatives own the place, and the house is full of photos. There is even one of her special cat!

The Legislative Buildings are worth a visit...older architecture and loaded with Canadian history that is actually interesting.

Visit red sand beaches...enjoy seafood with PEI's own potatoes, which are a main industry on the island.

AND - Cow's Ice Cream Parlour - homemade and local. It started in PEI, and has a few outlets in Canada, Halifax and Banff amongst them.

Couple of links:
http://www.gov.pe.ca/index.php3/

Tourism guide:
http://www.tourismpei.com/index.php3

Enjoy!