So the project will use a couple of ping sensors somewhat continuously for, say, 1 hour per day, and the whole thing will go into low-power mode for the remaining time. During that 1 hour, it will be using probably 60-100 mA current, and in the remaining time, it ought to use very little current. So alkalines will be insufficient, as they won't be able to satisfy the high-load times for long.
My current plan is to use a lipo that is solar-recharged.
But again, what I'm confused about with your note is why nimh would be preferable to lipo? Does it have something to do with the voltage that makes it work better with the arduino?