During our southwest road trip across 4 states, we decided to make a pit stop here at Pioneer Park and check out the supposed slot canyon right in the city of St. George, Utah. It turned out to be the narrowest slot canyon I've been in, and if you're passing through, this might be a fun stop for you to include.. but not for the claustrophobic!
Location: Google Maps
I wouldn't really consider this a hike, but it is a fun outdoor excursion for the family. The short slot is located right by the parking lot, and I could definitely see there being a line waiting to pass through the slot, since you can only fit one person at a time. Once you reach the end, you climb out and find your way down the slick rock back to the parking lot.