Finnish Nationals do not require a visa to enter Canada. They would require an eTA if arriving by air, but that is not your intention. As such you should merely need to present yourselves and your passports and the border and should be allowed to enter (following some degree of questioning by Canadian border guards.)
Equally, returning to the US should not require any additional documentation - I do recall however many years ago as a national of a VWP country being charged a fee to enter the US via land. It feels like there should be some way to avoid this if you are returning, but I'm not clear on the details.
As for the car, you should check with your hire company if:
- It is permitted under your rental agreeement to take the car to Canada
- The insurance provided will cover you and the vehicle in Canada