Rak mentioned the security theatre at the Canadian border [0]. I
agree, the current policy of requiring tests for fully vaccinated
travelers, even for short trips to the US, is absurd. But I wanted
to mention that there is a free COVID testing option that works even
for day trips to the States. While the rapid antigen tests are not
accepted at the border, nucleic acid amplification tests (NAAT)
are. One such test is the "ID Now" test, offered by Walgreens in the
US. While they advertise results within 24 hours, my experience has
been that the test results are sent to you within 1-2 hours (they
are sent by email).

When making reservations for the test online, they will ask you some
screening questions. Answer "yes" to the one about international
travel. That will get you the test for free. They don't seem to care
if you show a Canadian driver's license for ID. It's a
self-administered drive-through test, so it's very convenient. My
family has used it three times, once taken the day before crossing
the border, twice the same day. Each time the results have come back
within two hours.

Also be aware that even if the test comes back positive, a Canadian
citizen or permanent resident can't be denied entry into
Canada. Worst case, with a positive test result, you would be
required to quarantine at home for two weeks.

In any case, I think the Canadian government is under a lot of
pressure to remove the testing requirement for fully vaccinated
travelers, and I don't see it lasting much longer.

[0]: gopher://rak.ac/0/phlog/2021-11-08-Canadas-COVID-Security-Theatre.md