Never been to Southern Nova Scotia, just to Cape Breton up in the North, Bras d'Or Lake and environs, then Sydney and the ferry to NW Newfoundland (now That's a ferry trip!).
If you like rustic, small roads and steep grades, going around Cape Breton is a great trip. Cape Breton Nat. Park has some big campsites, but there are also private campgrounds with full facilities (never been to them though), especially around Bras d'Or.
And about crossing the border ... be sure you have the proper documentation. Passports, NEXUS cards are excellent, but apparently gov photo id and original birth certificates will still work if you're a US or Canadian citizen. I wouldn't want to try it though.
And the US Border guards can very picky about bringing in any fresh vegetables/meats. When we go south for the winter, the frig is empty until we cross the border into the States.