It's been a few years now since I vacationed on PEI, and that was mostly on the north side of the island in the Cavendish area. There's a couple gov't parks in the area and some real nice beaches - water is cold though *L*. Lots of nice walking trails and touristy type attractions such as the Anne of Green Gables park, museums etc. There are some nice golf courses on the island if they're into that. PEI is not a big place, so it's possible to pretty well cover the whole shebang in a week or so. We stayed in some beachside cabins, instead of hotels - more privacy/freedom. Here's a link that may be of some help:
http://www.gov.pe.ca/visitorsguide/
Hope they enjoy their trip, it's a nice quiet relaxing place as opposed to a party destination - lots to see and do, without the hangovers *L*.