You should not miss the restauurant at the Rossmount Inn near St. Andrews, which is about 45 minutes from the Maine - New Brunswick border crossing at Callais - St. Stephen. It is on your way to PEI.
I understand what edumont is saying about food in the Maritimes, but this is changing and the Rossmount is the restaurant that proves him wrong. It is really, really good and features local ingrediants at their best:
St. Andrews is also an excellent spot to stay overnight or stop for a break from driving. Great local art galleries, beautiful old houses, etc.
The other place that I wopuld hit is a restaurant called Dayboat in Hunter River, PEI. I haven't eaten there, but it is the restaurant I would try if I was going there (which I likely will this summer).
I hope you enjoy your trip!
I now live in Texas but I was in born in New Brunswick and have travelled many times to both PEI and Iles de la Madelaine. At the risk of offending Maritime Chowhounds, I will say that you may find it difficult to find good/great food in any of these beautiful places. I have not had any great food experiences in restaurants in the Maritimes and I have often been disappointed. That is not to say that the food is horrible, but it is not great and the fish and seafood are especially disappointing.
In my experience, the best restaurants are on PEI although the recent Quebecois "terroir" renaissance has probably made more headway on IDLM. "La table des Roys" has garnered some good reviews and I liked it when I was there last. My advice food wise is to keep your expectations low and enjoy any surprises that come your way.
You would think that fresh fish and seafood are easy to come by, but you may be surprised. Just a few years ago it was still difficult (NOT IMPOSSIBLE) to get anything other than batter fried fish and seafood on the islands.
PEI and IDLM are all about the scenery (there are great provincial and national parks), the beautiful and mostly empty beaches and the VERY friendly people.
My wife and I keep a fully equipped picnic basket and electric cooler in the car when we are in the Maritimes, local products/seafood are readily available at most markets. We have had our best meals dining al fresco overlooking the water and the sunsets on the Gulf of St Lawrence!
A few activities that come to mind:
If you are in Fredericton, NB on a Saturday morning go to the Boyce Farmer's Market, go early because it gets packed but you will find good food and great local people.
St Andrew's by the sea makes for a nice pit stop on the drive thru NB.
PEI is the home of the Malpeque Oyster and plump Island Blue mussels but you will be surprised to discover that they are available on very few menus, so seek them out assidiously, the best C'town restaurants will have them on their menus.
If you get to Nova Scotia and Cape Breton make sure to spend a few days following the Cabot Trail. Take your time and stop at most of the smalll villages. This a celtic music area as well as a strong french influence. If you have like to bicycle take a meandering ride on PEI Confederation trail. PEI is another place where you have to meander around rather than just go to the top attractions, take a sea fishing trip.