Tsukasa has been manly plenty of times for Sei, but he gets a weakness in the form of fast-moving amusement park rides. Normally this might seem like another flub on their date because he looked uncool, but this unexpected weakness tickled her heart and that smile says she thinks it's cute. She gets to turn the tables and has him blushing this time.
I love it when (would-be) couples like this get to turn the tables on each other, and this haunted house attraction seems like the perfect opportunity for Tsukasa to get back at her a little.