Driving over the Confederation Bridge was a good experience for me. Visiting the Ann of Green Gabel site was a good experience for the kids. Other than these, I did not find PEI as that attractive. If you can spare a few more days, I suggest you to visit the Cape Breton National Park (NS). It is really beautiful. I found The Fundy Isles, Manan Island, and the White Head Island (all of NB) are very good places to vist, too. You may need 10 days for all of these (driving?).
To plan the road, you may visit the websites of the Atlantic provinces' tourist ministries and requst visitor's guide. In the guide, you can find many families doing the Breakfast and Bed (B&B). Staying in these families also proved as a good experiences (you may need to call them early in the day to reserve a place).
