Good question,
A/C is not practical but I would have a good sized fan mounted up high. A better than average heating system if I could swing it and a hardwired, in dash, radio. I think these things should be standard already...
As far as perks go I would want a top of the line seat that can actually be adjusted (unlike the old 800's) and a thicker steering wheel. The old thin plastic ones slip in your hands in the summer. A low passenger-side 1st step and low hand rails. Lastly, a rear flor that slopes ever-so-slightly to the back to run the water out. I have seen this in another delivery truck and I thought UPS would do well with it.