Have your character show up at “x”‘s house at 7pm – dressed to the nines w/ flowers because he thinks he is going out on a date with “x”. However, when they get there they realize that they are actually there to babysit so that “x” can go out with someone else.
If your character is in high school, it could be that he shows up thinking he’s going on a date with the girl he has a crush on. (In a previous scene he thought she was talking about showing up at 7pm sharp for THEIR date, when she really meant so he could babysit her little bro/sister so SHE could go out on a date with Mr. stud jock or whomever.) He plays it off like he knew the whole time.
Or, it could be maybe that he has a crush on his teacher and he thinks she’s asking him out on a date, but she’s really asking him to babysit so she & her husband can go out.
I think it’d be a cute little plot twist for a romance or comedy.