All the comments above are good, but I would recommend reservations for both the July 1st week end and the first week end in August, which is a municipal/Provincial holiday in many parts of Canada.
My list of must sees: A)NS
1) Habitation Port Royal, the 2nd oldest European Settlement in North America
2) Blomidon Look Off, a sacred Indian look off over the Annapolis Valley
3) Chester, Mahone Bay, & Lunenburg, each unique fishing vollages
4) Halifax, the only city founded by Royal decree
5) Fortress Louisburg
6) Cabot Trail
7) Miner's Museum, and Ann Murray's Museum in SpringhillM
B) NB
1) Upper Saint John River Valley Edmunston to Fredericton
2) Kings Landing
3) Saint John
4) Saint Martins
5) Acadian Peninsula
6) Parlee Beach (where our summer home is located)
C) PEI
1) Anywhere on the whole Island in the summer
2) Kensington/Malpeque area
3) Cavendish, including Anne of Green Gables' house
4) Montague
These are but a few highlights, and all will be in Provincial information brochures, along with local parks.
As for New England, we stay either at Wampatuc State Park in Hingham MA for Boston, or at Salisbury SP for the beach. We love the White Mountains which are in both NH & ME. Acadia NP, not too far from Bangor is a must see, and you will be able to have a mini Rally with Gary RV Roamer who summers there.
There is just so much to see and do along the east coast, as there is all along your route to get east. Keep in touch as you head out and we'll try to help with suggestions.
Ed