TL;DR: Go with the Sigma 18-50mm f2.8 DC DN Contemporary for Fujifilm X! It is seriously the best all-arounder for travel and handles the 40mp sensor like a total pro. I was honestly so scared to try anything other than Fuji glass at first because I didnt want to risk my photos being blurry or having weird errors. I am usually the type of person who reads every single manual twice before even turning the camera on lol. But I finally bit the bullet and got the Sigma 18-50mm f2.8 DC DN Contemporary for Fujifilm X before my last big trip and wow... it was just incredible! The autofocus is snappy and it is so tiny that it doesnt weigh you down at all. For a trip to Japan, you are gonna be walking a ton and having one lens that can do it all is a literal dream. I used it for everything from tight shots of ramen to wide views of the temples in Kyoto. The f2.8 aperture is fantastic for those night markets too! If you really need a prime though, the Sigma 56mm f1.4 DC DN Contemporary for Fujifilm X is super sharp and amazing for portraits, but for pure versatility and safety on a trip, that zoom is the winner. I felt so much more confident knowing I wouldnt miss a shot because I was busy fumbling with lens caps in a crowd!

Building on the earlier suggestion, the technical performance of Sigma 18-50mm f2.8 DC DN Contemporary is incredible! To really push that 40MP sensor tho, you must examine the Sigma 56mm f1.4 DC DN Contemporary.

• 18-50mm: Unmatched versatility with stellar MTF data for a zoom.
• 56mm f1.4: Superior resolution floor, outperforms many native Fuji primes. Check the data on OpticalLimits for raw benchmarks, its a total game changer!

Regarding what #2 said about technical performance, I agree, but be careful with costs tbh. Looking for one lens or primes for Japan? Whats your budget? I can help narrow it down.