Fundy National Park is beautiful-hopefully it's in better repair than than it was about 10 years ago. They have a number of campgrounds in the park-big rigs can fit at Headquarters CG. If you go there be sure to visit the bakery in the town of Alma, located at one of the park entrances. Go to Hopewell Rocks for formations but watch the tides. See the tidal bore in St. John or somewhere near there. There are also some amusement type parks in that area. On PEI head to Charlottetown, the capital. It's just a pretty place with shopping and dining. Head to the College of Piping in Summerside for a performance by the students or a guest performer. Nova Scotia has beautiful scenery and the Alexander Graham Bell museum in Baddock. How much time you have and what you like to do will help decide where to go.
I'm a planner so I always make reservations. I don't think the crowds there are any worse or better than most tourist areas. Keep in mind that Canada Day on July 1 is similar to our July 4th-locals come out of the woodwork to celebrate.