Halifax is an interesting small city with a lot of history. The hop on/hop off was a good way to see it and decide what you wanted to see in more detail. The Citadel, the park, and the Museum of the Atlantic are all worth seeing.
If you've never seen the Bay of Fundy, that would be my first recommendation. We've been here many times and this time we just walked to the Farmer's Market. It's a one-of-a-kind farmer's market and well worth the short walk from the port.