Ottawa's most famous landmark is the Peace Tower, so famous that having wedding pictures with this symbol can seem petty much too obvious. If you were getting married in Washington DC you'd want pictures with the Capitol and in Paris you'd want...the trick is to make it original. But how?
A lot of the time I will go to the Parliament Buildings with couples late at night. How late? Try between 1 and 3 in the morning. After you've said good bye to all of your guests, after everything has been said and done, you might just want to collapse - but then you might also want once in a lifetime night portraits on the grounds of a property that is open to the public. Free Access without tourists and with police security.
There is one spot where I can make pictures showing the entire Parliament buildings, and then I go to the other side of the Centennial Flame and photograph the same pose.
The Fence around Parliament Hill also has promise for original portraits.
And there are other places with views of the Peace Tower like this view from beside the Chateau Laurier,
the view from the Mackenzie King Bridge (between Rideau Centre and the NAC)
and from the Rideau Canal in front of the Ottawa Convention Centre.
You can still go to Parliament Hill during the daytime, the main challenge is a bright and sunny day (and who doesn't want that) which can be a challenge for squinting and a quantity of tourists but can give you the feel of Ottawa.
You can also go behind the Parliament Buildings and pose with statues or have the Library in the background.