If you're camping on a border lake and planning to day trip to the Canadian side, do you need to have a passport or just RABC day permits?

Technically you need RABC permits for each member of the party, Ontario Park Day permits (if the shore you're landing on is in the Quetico) and passports for reporting to customs when you return to civilization.

Getting an RABC permit is the process of going through CANADIAN customs without having to stop at a customs office on the highway. It allows you to legally cross the border remotely. As mentioned above, each person in the party needs one. Youth may be included on an adult's permit.

RABC permits are good for one year. There is no such thing as an RABC day permit.

Day permits would be required by Quetico for you to be in the park legally.

When you return to the US side and you're leaving the wilderness, you are required to stop at the US customs office to officially and legally re-enter the United States.

There may be an additional point or two that I missed, but to be legal, it's not as easy as you might think.

And iirc, the Q day permit is called a vehicle pass - same one if you drove into the park at Dawson Creek. A little quirky.