A really sturdy tripod is the most essential accessory, with cable release.

As for lenses, I have 110, 65, and 50, and have just acquired a 250. My main use is for landscapes. I can carry them all and the tripod, but if I wanted to cut weight and just take two lenses, it would be 110 and 50. If I could only take one lens, it would be the 65mm.

I have one image with the 50mm (not the ULD version) which shows a lot of distortion at the edge, but need to shoot some more before drawing conclusions.