It's worth strolling through the government district of Ottawa if you haven't been before (and the weather is alright). It's fairly small and easily walkable. Free guided tours of the Centre Block of Parliament (including the House of Commons and Senate chambers) are available. Also stop by the National War Memorial, the Supreme Court and the Peace Tower, if only to take a look from outside. Taxi to and from the airport won't take more than 20-25 minutes each way.