I have taken 90mm with me on 2-week trips but seldom use it. I once took along a 500mm lens that I had just gotten, and I did have fun exploring its use. But atmospheric haze has an unfortunate effect on the distant subject (and where compression of perspective may be interesting), while nearer subjects seem to suffer for that flattening, are extracted from their context, and IMHO better treated by moving close with a wider lens. If I were taking the SLR anyway, the 500 mirror is a minor additional burden, so I might take it again, but at the time I had only that one lens for that body.
I'm tempted to say I'd choose the Bronica RF645 plus its 45 and 100mm lenses if limited to two. That would be very satisfactory, I think, as it's quite compact... about the same size as a Leica M, though 1" taller. But several other kits would be satisfactory too, depending on how dedicated is the trip to the pursuit of photos, transportation details, etc.
Interesting thread...