Congrats on the upgrade! The a6700 is a beast. Honestly, if you're trying to keep costs down for your Japan trip (save that money for the ramen and sushi!), the Sigma 18-50mm f2.8 DC DN Contemporary is the absolute king of value right now. I have used it on plenty of trips and the size-to-quality ratio is just insane. It is way cheaper than the Tamron and much smaller. While the Sony E 18-135mm f3.5-5.6 OSS gives you more reach, you are gonna miss that f/2.8 in those narrow Kyoto alleys at night. IBIS helps, sure, but it wont stop motion blur if people are moving around. The Sigma is so affordable that you might even have enough left over to grab a cheap used prime like the Sony E 35mm f1.8 OSS for even better low light, though the Sigma usually does the trick for most things. Another tip for the budget-conscious: check the used market for the Tamron 17-70mm f2.8 Di III-A VC RXD. If you can find a used copy for a good price, the stabilization is nice, but honestly, it is pretty bulky for long days of walking. For a walk-around lens in crowded places like Tokyo, the Sigma is less intrusive and keeps your kit light. I would go with the Sigma and spend the savings on a JR Pass or better food. It is easily the best bang for your buck lens for Sony APS-C right now, and you wont regret the portability... definitely the smart choice if you want to stay within a reasonable budget.

Saw this pop up and had to jump in. Since you have the a6700, that IBIS is a total game changer for travel. Honestly, for Japan where you are doing a mix of everything from tight shrines to wide cityscapes, the Sony E 18-135mm f/3.5-5.6 OSS is a total sleeper hit. I know people get nervous about the f/5.6 aperture on the long end, but the a6700 sensor is really clean. You can easily bump your ISO to 3200 or even 6400 and get great results for street photography. The weight savings alone makes it worth it when you are walking 20,000 steps a day through Tokyo or Kyoto. If you really want the absolute best optics though and have the budget, the Sony E 16-55mm f/2.8 G is the pro-level choice. It is tack sharp and built like a tank, though it is definitely more of an investment. My practical advice? Grab a used 18-135mm for the versatility during the day, and then use the money you saved to pick up a tiny, fast prime like the Sigma 30mm f/1.4 DC DN Contemporary for those moody night shots and food photos. The f/1.4 is gonna give you much better low light performance and background blur than any f/2.8 zoom ever could. That combo is way more flexible for a long trip imo. Have a blast in Japan, it is a photographers dream!

I went through this exact struggle last year. I ended up picking the Tamron 17-70mm f/2.8 Di III-A VC RXD because I wanted that constant f2.8 for night shots in Tokyo. Ngl its definitely bulkier than the Sony E 18-135mm f/3.5-5.6 OSS, but the low light performance is worth the extra weight. Having 70mm at f2.8 gives way better background blur for food shots than slower zooms imo.

I have been shooting with Sony APS-C gear since the original NEX days and honestly, if youre heading to Japan, you want gear that just works without you having to fuss over it. I took the a6700 to Kyoto last spring and was super satisfied with a couple of options that often get overlooked because people fixate on the newest third-party stuff.

• The Sony E 16-55mm f/2.8 G is the one I always reach for now. It is a bit pricey, but the build quality is on another level. Its weather-sealed which totally saved me during a sudden downpour in Gion. The images are sharp as a tack and the f/2.8 stays constant, so you dont have to worry about your settings jumping around while you are zooming in for a shot.
• Another reliable workhorse is the Sony E PZ 18-105mm f/4 G OSS. I know its an older design, but the internal zoom is a lifesaver because it doesnt extend. This keeps dust out of the barrel and makes the lens feel much more solid in the hand. The f/4 is plenty for street photography since the a6700 sensor is so good at higher ISO. Both of these have that G series reliability that makes them worth the investment for a big trip like yours.