United is currently advertising a 2 for 1 sale to various Canadian destinations from US cities. Fares start at $208 RT ($104 one-way) plus tax from Chicago to Toronto. Denver to Winnipeg is $380 ($190 one-way) plus tax. Not especially great deals if you are traveling by yourself, but a decent deal if traveling with a companion.
And interestingly, in order to get the discount you must call United reservations at 1-800-864-8331 and mention the "Canada Offer". Phone reservation fees will be waived. Fares must be purchased by July 5. A 2 night minimum stay is required. Travel valid between now and September 30th. This is yet another example of how airlines are forcing you to deal directly with them and not through third party web sites in order to get the best fares.