Do you like driving trips? If so, Newfoundland is amazing. There's some great nature activities and some fascinating historical sites. The North Atlantic Aviation Museum in Gander was the sleeper hit of the trip for us in the museum/historical site category. The Rooms museum in St. John's has some interesting bits related to the Titanic because the first distress call was picked up on Newfoundland, among other things. And the highlight is L'Anse aux Meadows. Lots of other interesting places related to the First Nations and early settlement of the island.
For outdoors activities, the national parks throughout the province are wonderful. I'll just mention Gros Morne and Terra Nova to start. Go see the fossils at Mistaken Point (reservation for guided hike required) and the birds at Cape St. Mary's.
That's just a taste. You'll have to cover a lot of ground, but I found it totally worth it. We flew into St. John's and out of Deer Lake, getting as far north as Red Bay in Labrador (via the car ferry) and L'Anse aux Meadows on Newfoundland. We were there in July/August and the weather was cool. Very few crowds, with the exception of the time we ran into a cruise ship tour (but a small museum was open for them that we wouldn't have been able to see otherwise) and also when our whale watching tour out of St. Anthony had to be rescheduled because a cruise ship need to use the dock to process passengers as they tendered in from the ship.
I highly recommend Newfoundland.