Walking across North American borders: from Canada to Panama
There are only two ways you can go through all mainland countries in North America (you go through Mexico but not Cuba). From Canada to Panama or the opposite: from Panama to Canada.
This was a fun trip for me as I have actually driven from Panama to USA and back, camping out in each country on the way. Have also driven to Canada several times from US but not on same trip.
Thanks for an interesting idea!