Just one lens is really difficult. It is hard not to take the 105/2.5 along, for what I'd call "instinctive shooting". Something wider is sometimes needed. My favourite, additional companion (I've used it again over the weekend) would then be the Tamron 45/1.8, not only because of the "complementary" FL and technical features (AF, VR, close focus, low coma...), but also due to the IQ and, in particular, the bokeh which is much better than most standard Nikkors' (Noct excepted). In fact, you may call it the poor man's Noct (ok, FL is really different, and 1.8 is not 1.2, but 1.8 is fully usable, night and day).