Nikon 35 f/1.8 DX - small, light, and a good mid focal length, but my body is usable at ISO 12800 and quite competent at 6400, so I don't need faster.
The other primes I'm bringing along are the 85 f/1.8 and 105 f/2.8 VR Micro for night telephoto, otherwise it'll just be my 16-85 f/3.5-5.6 VR zoom on my body most of the time. Almost nothing outside of AK requires a really long lens, after all, and EV levels don't tend to favor using an f/2.8 zoom (it's either very bright or quite dark, without much in between).
Money and packing space no object, I'd bring along a Coolpix A or similar large sensor compact for my indoor shots. But that means another set of batteries, chargers, and expense, for something that has very limited use for me outside of once or so a year.