Honestly, the most important thing for that camera is extra juice. The battery life is kinda weak for a full day in Tokyo. I would suggest picking up a Wasabi Power Battery 2-Pack for Sony NP-FW50 so you aren't stuck with a dead camera. Just be careful with generic SD cards too; make sure to get something reliable like the SanDisk Extreme Pro 128GB SDXC UHS-I so you dont lose your photos.

Like someone mentioned, power is key, but unfortunately cheap filters ruined my Osaka night shots. Are you shooting after dark? TL;DR: skip filters, just get an Expert Shield Screen Protector Sony a6400 Glass.

^ This. Also, skipping the cage is definitely the right move for street shots in Japan! You want to be as light and nimble as possible when you're exploring. The advice so far about fast cards and extra batteries is 100% spot on for the a6400. Technically, you should look into the USB Power Supply feature. Its fantastic because it lets you power the camera directly via a power bank while you're actually using it! I always carry the Anker PowerCore 10000 Portable Charger in my bag. It saves so much stress when you're out all day and those tiny NP-FW50 batteries start dipping. Since you're worried about weight and bulk, maybe swap the neck strap for a Peak Design Cuff Camera Wrist Strap Black. It is amazing for keeping the camera secure in your palm without a long strap getting tangled in your bag or caught on people in the subway. You are gonna have an incredible time in Japan, that camera is such a fantastic choice for travel!

@Reply #1 - good point! Batteries are definitely the priority for those long walks. Just saw this thread... I was quite satisfied with my setup during my last trip to Kyoto, and keeping things light made a huge difference. Technically, you dont need the most expensive gear, but you do need gear that meets the cameras specs. For example:

• SD Card Write Speeds: The a6400 writes 4K video at 100Mbps, which is only about 12.5MB/s. However, for street photography bursts, you want a card that handles faster buffer clearing. I use the Lexar Professional 1066x 64GB SDXC UHS-I V30 and it works well without complaints. Its 160MB/s read speed makes transferring photos to my laptop much faster at the end of the day.
• Ergonomics: The stock strap is really basic and kind of rough on the neck. I am happy with the Peak Design Slide Lite Camera Strap SLL-BK-3 because of its seatbelt-style webbing and the anchor links. Each anchor is rated for 200lbs, which is massive overkill for an a6400, but the peace of mind is worth it. It lets you tuck the camera behind your hip so it doesnt swing around while you are walking through crowded areas like Shinjuku.
• Dust Management: If you are switching lenses at all, you will eventually get sensor spots. I use a Giottos Rocket Air Blaster Large to blow off the sensor and glass. It is a simple, mechanical solution that works better than any fancy brush, and I have no complaints about its portability. Focus on these basics first. Skip the cage, honestly. It just adds unnecessary weight for street photography and the a6400 grip is usually enough for casual shooting.

Great info, saved!