Whether or not you have in-bound immigration or out-bound immigration (to the US) will impact your planning.
On a Saturday, it should not take more than 30-40 mins in a cab or Uber.
If it's sunny, go to Place Jacques-Cartier or Notre-Dame Basilica. Walk around Old Montreal and soak in the atmosphere.
If it's raining, go to Montreal Museum of Archaeology and History.
All three places are near each other and are perfect for a first-time visitor.
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