With the combined power of Post-Nut Clarity and Showerthoughts, I have deduced that the swimsuit shopping chapter will come before Ch50. The gag is that Ooyama has a selection of suits, and Akutsu-san actually picks out the one he wants most (accompanied with a panel of him having a full body delusion). Unfortunately, he does not get to actually see it, and the end gag is that Ooyama was never invited to the beach trip.
The beach trip arc/episode/chapter happens about one year later (if the manga actually continues this long). The gag is that Akutsu-san is almost picked up by our stereotypical, trope-ic beach-playboys. Ooyama intervenes, and the end gag is either
A: It turns out the playboys are indeed just childhood friends or something, or even strangers asking for help/directions, and Ooyama yet again has another misunderstanding
B:
Akutsu: "This guy here is my boyfriend, you dweebs really think you can stand up to him?" (Ooyama showing some muscle definition because of both diet and Akutsu-raining)
"Shit, what did I just say?"
C:
Ooyama: "She's with me." (tl;dr ooyama temporarily becomes chadyama again)
Akutsu (inner voice): "Shit, why did he have to say that? Why did he sound so cool?"
Ooyama: "I'm so glad I'm alive/I'm never doing that again."