Crossing US/Canada Border for less than 24 hours

In the early 1990s, I lived in Windsor, a Canadian border town. Back in the day, every Saturday morning I would cross the border into Detroit. I would drive a couple of miles on I-75, get off at Mack Ave, fill up my car, turn around, and come back to Canada. At the border, on both sides, I would truthfully answer that I was crossing for 20 minutes to get gas. Even with paying the tunnel crossing fee, it was worth it.

Granted, things are different now, but still people on both sides of the border routinely cross for less than 24 hours. I have family and friends in Windsor and Detroit who do it at least once a week.
When asked your purpose of visit, tell the truth that you're going to see a tattoo artist.
Good luck with your tattoo.

To address the question of monetary spending limits, they only apply to goods that you bring back to Canada, as clearly stated in the customs declaration form. Services paid for and received outside the country are not subject to any limits.

My relative made a less-than-24-hour trip from USA to Canada while carrying a large amount of cash (~$600). Canadian border officials were not concerned with his entry into Canada. However, upon his return into the United States, US Customs became very suspicious of his brief visit, and after determining that he was carrying a large quantity of cash decided to detain him while they conducted a full vehicle search. Of course they found nothing, as he was not engaged in any illicit conduct, but the ordeal was both stressful and time-consuming. I would recommend bringing minimal amounts of cash into the United States so as not to arouse any suspicion of ill intent.