Apply for a Canadian passport, if your passport has expired. Present your Canadian Passport along with your Green Card for access to cross the Canadian-American borders.
Apply for a Nexus Card. The only thing to take into consideration with this option is that all the enrollment centers where you interview after the application is processed are in the states bordering Canada. If you cannot easily make it to one of these enrollment centers, then this would not be a likely option. However, if you are living in a bordering state, this would be a rather cheap solution, as the Nexus card is only around $50.
Apply for a Global Entry Card. This costs about the same as a passport ($100). Visit the website in the resources for a an application center near you.
