actually I would say that flash and tripod are quite essential for a holiday shoot, if you're intending to come back with WOW shots.
If you want to shoot with long exposures in order to smooth out the water surface, what do you need? Tripod.
If you shoot against a slightly bright background and need some fill light, what do you need? Flash (or reflector perhaps).
Japan's night scenery is pretty fantastic, especially around a large metropolis like Tokyo. You'll regret not having a tripod!! :devil:
on my recent trip to Bali, I brought along 18-135mm kit lens, 50/1.8 prime, travel tripod and SB400 flash. Very compact.
The body + 2 lens and SB400 could all fit in Crumpler 5million$home, and the tripod slung over shoulder. Not very heavy as well.