Landing in Canada and, EVEN MORE IMPORTANTLY, coming back into USA, is much better done if planned and well prepared than because of a fuel diversion.
Normally you will have to choose Port of Entries (places with custom facilities open to GA at the date and time you land) in Canada, and back in USA. You would need a flight plan with an estimated crossing time. You will need to advise CANPASS about your intentions and communicate your manifest before you fly there, and you will have to file some forms (look up EAPIS) for the USA counter parts. You will need your passports, in theory (nobody asks for it) your radio operator and radiostation licenses, and a decal for the current year for USA customs. In other words, I would consider landing in Canada, in an unplanned situation, only in case of emergency.
Now, if you just overfly the foreign territory, ON A FLIGHT PLAN and under radar control, then the worse that can (and will) happen is that the registered owner of the tail number will receive a bill for the Canadian ATC services (radio calls, etc) that you have been consuming.
Best of success with your adventures!