Who knows? Other than Ria.

They stared at the box, waiting for something to happen. "...I opened it, but nothing happened," Kazuma grunted.

"Boooring!" Aqua pouted. "Wiz will hear about this! Sell us a defective item will she!?"

"Maybe we did go, but just don't remember?" Fio suggested.

"Seems more likely that that's just the bullshit reason given by whoever gave it to Wiz for why they couldn't prove it works," Cremea huffed. "It's a pretty darn convenient excuse..."

"Um, guys?" Darkness asked, going over to the window.

"Eh?" Xylixia asked, turning towards Darkness, and pausing as she saw what she was getting at. "Wait, it's dark out! It was still early afternoon when we opened the box!"

"Wait, so we really DID go to this Japan?" Megumin asked. "Barely seems real without any memory of it..."

"...hey Fio, what's that thing next to you?" Cremea asked, pointing, and causing Fio to look down at a large briefcase at her feet.

"Eh? That wasn't there before!" Fio remarked, fiddling with it, and eventually figuring out how to pop it open... Before closing it, going red in the face. "Oh, it's nothing, I guess I just got some souvenirs from the trip, but I can't for the life of me remember how!"

"How did you get souvenirs without money?" Kazuma deadpanned.

"Isn't it obvious? Her class IS a thief after all," Aqua sniggered.

"I wouldn't steal from any legitimate business!" Fio huffed.

"...even if you knew you could get away with it since the trip had an escape ticket?" Darkness asked shrewdly.

"No!" Fio denied immediately.

Aqua meanwhile found a keychain in her pocket, and looked at it in bemusement. "...oh well, guess I'll give this a good home!" she said cheerfully.

...

Kazuma laid in bed, wondering which of his harem would come in tonight, seeing as he had an open invitation for them to decide among themselves. "Guess I did everything I needed to do... I hope," he remarked.

"I could tell you!" Ria noted, popping down face first through the ceiling with a wide grin.

"Ack!" Kazuma jumped. "Damn it Ria! ...wait, how would you know how the trip went?" he asked.

"Duh, I went to Japan too!" she replied, sticking her tongue out.

"Then... How do you remember?"

"Well, I doubt the box could wipe my memories, but I played it safe and made my own way over," Ria shrugged. "I literally cross the multiverse for the sake of entertainment, so it was hardly a challenge to tag along via a different route!"

"Alright then, tell me what happened!" Kazuma sat up eagerly.